For some reason I was thinking Terrak was 109... :/Infernape and Terrakion both have 108 speed lol.
Why is Thunder Punch a must? I really don't see the reasoning since Infernape's main problems are Azumarril, Mega Pinsir, Talonflame, and the Lati@s. the first three all have priority and basically require you to predict to use it otherwise the moveslot is wasted, (Ape also has fire moves for predicting Pinsir lol) and U-Turn is how you get Lati@s killed via pursuit from other dude. U-Turn is a MUCH better option.
Anyway, Thunderpunch allows Ape to, yes, hit counters on the switch. Tentacruel, jellicent, Azumarrill, and Talonflame are easily worn down by a few Thunder Punches, enabling Ape to break through it's own counters. Much of the time when using Ape, Thunderpunch will have the highest reward:risk ratio, able to easily beat down its counters while infernape looses a little bit of health if it mispredicts to something like a choice locked scizor bullet punch.