Wiki says
バーストダメージアップ Lv1毎に+1.5% (バースト倍率に加算) 例:バフLv100 SSR神姫のバースト倍率が5倍(500%)か 6.5倍(650%)に
Google translates to
Burst Damage Increment + 1.5% per Lv 1 (Add to Burst Magnification) Example: SSF Shinki burst magnification increases from 5 times (500%) to 6.5 times (650%) with buff Lv 100
(Not sure why google translates SSR to SSF but the original Japanese text says SSR. Shinki = himes)
And after checking my own fight, I can say that 100 into double attack rate is not 100% double attack rate.