I don't really see the evidence, for why she has become "stronger"... The way I see it, Sakura was never allowed to go all out in battle, because she has to follow the medical ninja rules, meaning she must not die in battle, and thus cannot take big risks in battle. The byakugo jutsu allows the medical shinobi to ignore this rule, because she can restore herself. Thus Sakura hasn't become any "stronger", she just hasn't been able to participate on the front line until now. I dno why people think the strength is her power-up. It's a really pathetic power-up :| she had enough before as well. Killing her targets even more, wont do any difference. Dead is dead.....