No, they're flat values.Are the stat points a straight percentage? IE one point is a one percent increase?
Item stats used to be percentage based, but it seems they've changed that in XV2. +5 ki or stamina from QQ bangs seems to always give 3 bars of each, regardless of how many points you put into either. I think it's probably the same for the attack stats, which is likely also why they seem to make such a tiny difference on max level characters.
To land ultimates consistently you have to land a stamina break first. I suggest looking up the stamina break combos for whatever character you're playing (if they're available - if not, you might have to experiment in training mode for a bit).What's the best ultimate to use at the end of a combo? Final flash and perfect Kamehameha keep missing.
For what it's worth, x4 Kaioken Kamehameha is one of the few that doesn't need a stamina break to land. Kaioken Kamehameha (the super attack version) is also a guaranteed combo ender that does a lot of damage for 1 ki bar.