Going from 300 to 365 is about a 31% dmg boost for most weapons and 32-34 enemies, not sure about 35. Going from 331 to 365 is around 18%.
The thing is though I don't think most people are really having much difficulty in POE 32/34 or the rounds of 1-5 POE 35. I've done all of them in LFG pugs with 331 weps and never once found myself thinking "man if I just did 20% more dmg this would be so much easier!". It really was just running away, regen-ning, rezzing, and jumping back in. Titan bubbles, hunter invis, and warlock rez were much more clutch on mine phases than extra damage.
There are only a few specific fights where a few specific weapons make a huge difference. A 365 gallahorn is super useful on some of the critical target waves since you can fire across the map while trying to avoid the million elites sniping you. A 365 sniper (arc/void) is super useful on skolas burn weeks. I guess you could argue that a shotgun is useful on the judgement shield boss in POE 32/34, although if you either do the super burn or correctly kill the right mob its really not that important.
So far I have 8 etheric lights in my vault and have only ascending a single fatebringer, and two snipers (arc/void). I don't play ToO though so if I was doing that I'd probably burn some more but even still the loadouts are pretty standard across ToO. I'll probably do a VoC next although the last two weeks there haven't been many solar burn rounds in POE (none this week afaik).