I got that info from here. I wonder when it changed. I always thought that being at final ascension but not maxed got you to 3/3/6/3 not 5/5/10/5. Huh. Interesting.
EDIT: Oh. I think I get my mistake. To get costume stat bonus the COSTUME must be maxed. And to get to last ascension on costume you will need to ascend base hero to final level (4-1 at least). Got it.