I personally don't think Ace's bounty will be higher than 300mill...
I mean, one with bigger bounty doesn't automatically strong.
Kuma only have 296mill, and Jimbei only 250mill. But those two are part of the 7 pirate lord. And we've seen how strong any member of the 7 pirate lord are (unfortunately for hancock, she met with Luffy, a super idiot that has no sexual desire. If it was Sanji that flew to amazon, hell be a statue with one mero-mero attack).
Maybe 270mill - 280mill will be more reasonable.