One of my favourite champions due to available skins in League of Legends is Corki. He's a Yordle with an awesome handlebar moustache who flies around in a little plane. I have a skin that turns his plane into a Red Baron biplane which is just awesome. I've always liked playing Corki and he's recently received a bunch of personal and item related buffs. He was probably the top AD carry at Worlds and they kept bringing up on the streams how he uses a different set of boots than most people at his position. Most champions use the boots that make you autoattack faster since most of their damage comes from autoattacking. Ezreal has a build that uses cooldown reduction boots and that was really surprising to me. I have to assume all these pros know what they're doing when they're building the spell penetration boots, but I wanted to test and math it out myself to see what was going on.
I've played a few games recently as Corki, and I looked at the match summary screens afterwards. It seems like my damage done to champions was about 67% magical or so. 10k physical, 22k magical, 1k true... that sort of split. So the spell pen boots would be amplifying 2/3rds of my damage and the autoattack boots might be amplifying 30% if they're lucky. It really feels like they have to be better!
What does the spreadsheet say? The spreadsheet really makes it look like I should be doing a lot more autoattack damage. I think I'm failing really badly at modeling how Corki does damage. The problem is he has a huge range spell so often he'll be hanging in the back of the fight not able to auto attack at all. Also his spells do big area of effect damage so it's possible to hit multiple people at the same time with the spells and not so much with the autoattacks. I even went so far as to put in a full AP build but it couldn't come out better than an auto attack based build with trinity force.
Ok, so my spreadsheet is a disaster here. What about logic? Well, with that much of my real reported damage being magical it really seems like the spell pen boots are important to have. I should likely also run spell pen runes and masteries as well. Well, probably hybrid runes if I ever got around to buying them. But I think the big thing Corki has going for him is his ability to do damage at the start of a fight with his long range spell, but to still do a ton of damage up close in a skirmish or as cleanup. He needs to build a lot of standard AD items in order to make that happen. His passive doing 10% of his AD on each auto attack is pretty strong, but only if you're building normal stuff.