You're assumption is pretty much right.. Larger applications like 1/8 electric may benefit from 30-40C cells. Most electrics I think will be perfect with 20-25C packs.
I run 3 packs in 2WD electric, a 16C Trakpower, a 20C Orion Platinum, and a 35C Reedy. The 16/20C packs both work just fine and I have great power through the run, no complaints. However - the 35C pack allows me to clear a certain triple at the track I run at, and I cannot do that with the other two packs. My consistent laptimes are also a little faster. The 35C packs has a little more punch, everyone I've let run it thinks the same thing.
Even though the 35C has more rip to it I would still suggest saving a few bucks and running a 20-25C battery. The extra cost doesn't give that much benefit. But when the cost comes down there will be no reason not to buy 30+ C batteries.