On the weekend I picked up three rings with +35% bonus experience. I debated a bit about how much killing power I was willing to lose in order to get 70% more experience and was leaning towards all of it. 70% is a really big deal! It then turned out you can only wear one hellfire ring at a time (legendary items are unique equipped even though it doesn't say so in game) which made the debate a little moot. And now after reading some more I'm starting to wonder if the first one is even terribly good. You see, bonus experience stacks additively, not multiplicatively, and I already have an awful lot of bonus experience.
First, apparently at some point the Nephalem Valour buff got changed to include experience on top of gold find/magic find. (Looks like it was patch 1.0.4 when they added in paragon levels, which makes sense.) So with a 5 stack of the buff as my only source of bonus experience the ring would take me from 175% to 210% experience, or a gain of 20%.
Next up, the monster power boost. This last patch massively buffed the bonus experience gained for fighting on a harder difficulty level. I've been fighting on monster power 7 recently which gives 375% normal experience. Tack on the neph buff and I'm up to 450% experience. The hellfire ring on top of that brings me up to 485%, which is only a gain of 7.8%.
Now, apparently you can stick MF/GF/XP items on your follower and get 20% of the bonus as spillover from them. Sticking one of my three rings on my scoundrel friend has to be right. That's only a 7% additive bonus, which along with the MP7 buff and the neph buff is 457%. My own hellfire on top of that is 492%, or only 7.7%.
You can also get a socketed helmet for more bonus experience. My current hat doesn't have a socket, but what if it did? I could probably afford something like a 27% ruby, which brings my pre-ring number all the way up to 484%. Hellfire makes it 519%, which is only a 7.2% boost.
Now, don't get me wrong, 7.2% extra experience is still something. But it's not the massive 35% I thought I was looking at. I can totally imagine situations where a real ring would be better than a bad hellfire ring when fighting at MP7. Fortunately for me one of my three rings is actually insane. Randy pointed out an interesting stat website that breaks down your stats by item and it turns out it thinks my hellfire ring is my most damaging non-weapon slot item. It's more than twice as good as my other ring! And it has better defensive stats than my other ring. And it has an empty socket I haven't made use of! It doesn't have attack speed or crit chance which hurts my stun locking ability but I was able to play in MP7 with it so I can't be that big a deal.
Assume now that I want to efficiently farm for experience, not infernal machine keys. Getting more paragon levels to get more stats and more magic find is going to be pretty useful. Also assume I find a build with better initial damage so I can actually kill trash mobs. (The wicked wind spec can lock down and kill MP7 elite packs but it takes time to get going each fight. I know there are better initial damage specs like archon, meteor, blizzard, or maybe even disintegrate to play with.) What monster power level should I play on for optimal efficiency?
There are going to be two factors at play in this calculation. The first is the amount of time spent just walking around finding things to kill, or loading new games, or clearing out my inventory. The second is the ratio of monster health to experience earned. Raising the monster power level isn't really going to increase the downtime from doing a single run. (Well, you get a bigger magic find multiplier so I guess you'll spend a little more time dealing with items?) It is going to increase how long it takes to actually kill things. The formula for experience per time is then going to be X/(D+H*Y) where X is the experience multiplier, D is the downtime constant, H is the monster health multiplier, and Y is a constant based on how long it would take to kill things at MP0. I have no real idea how to estimate D and Y but the absolute size of them doesn't matter at all. What matters is the relative values of them. Estimating is even harder when I don't know what spec I'd be doing it in and have never played it! I feel like it should probably be well above half my time spent killing things? I'm going to pretend 30% downtime and see what shakes out?
Even with the hellfire ring this function maxes out at MP0. Ok, how much downtime do I need before a better MP level makes sense? Well, with D=.9 MP4 becomes the best. With D=.8 MP1 is the best. So, yeah... Unless all my time is spent running around looking for stuff to kill I should stick to the lowest level possible. Note that if your primary goal is finding keys then you actually are spending a lot of time just running around. And the key odds function increases much, much quicker than the experience function does. (Going from MP2 to MP4 will double your key odds but only gives me 24% more experience.)
The thing to point out here is that sometimes increasing monster health doesn't increase time to kill things. If I kill most everything on MP0 instantly then giving them all 50% more health probably doesn't slow me down much at all and the 10% experience bonus is going to be worth it. So realistically the 'optimal' choice is going to be finding a point where I can still trivially murder practically everything in my path. I feel like my gear is starting to get good, so I wouldn't be surprised if an archon or disintegrate build would be doing just that at something like MP2 or MP3. With my wicked wind spec giving all the monsters more health gave me
enough time to ramp up my damage and get infinite resources back, which is another factor to keep in mind. I feel like with that spec I want to find the point where I can lock in a freeze, cast a bunch of spells for a bit, and watch everything explode. MP6 is probably right for that. But it's almost certainly going to be faster to use a different spec with no ramp-up time at a lower MP level. And my poor hand will love me for it!
The hellfire ring also gets relatively better at lower MP levels because it's adding in with a smaller MP bonus. Basically, it is going to add 35% of the base monster experience no matter how much health it actually has. So the more wimpy monsters you can plow through, the more experience the hellfire ring is going to add. So it feels like if you're playing on a low MP level then you really want to get a hellfire ring (and a ruby in your hat) but if you're playing on a high MP level then you probably just want whatever gives you the most stats. Or you want to get lucky and get a hellfire ring that happens to have a ton of good stats too.