you are correct that with a zeta lead, 5 resists would mean 25% tm.
BUT, remember what team r2 is on. It holds CLS, C3, Han, Chewbacca. C3PO gives 10-40% potency CLS has a special that inflicts tenacity down. Everytime Chewbacca assists he inflicts tenacity down. Consider that both callers to assist (CLS and Han) have moves that pretty much mean they take two turns in a row, meaning double the chance to inflict tenacity down.
Your best chance for a mass resist is to have R2 move first on your team. Han's Shoot first automatically lowers your resist chances.
So yes, a low potency R2 could get up to 25% tm for his team. But the odds of that actually happening aren't great. I'd question if it'd be worth modding your team around, especially considering that stun on basic is rather useful (or maybe not, with all the immunities going around) and you would just be lowering your odds of that working.
Also, who are you trying to fight these rebels with? Just thinking of a few recent teams off the top of my head, r2s potency probably won't even matter.
JKR has Yoda- tenacity up
Padme teams- immune w/prot up
DR- ferocity lowers tenacity
GG- actual normal potency check. GG/Magna may have bonus tenacity based off target lock. Droideka may have tenacity up
GS- Rex brings tenacity up
point is- some teams will almost some form of immunity that tenacity down from chewie may cancel if it hasn't been cleansed yet. Or have low tenacity so your potency is irrelevant
