Those will be taken into consideration once the information is released, however the V12 in the Ferrari is known to be extremely flexible.You're ignoring weight, traction and gearing for starters. Not to mention the usable power band of the two engines could not be more different.
The thing is, at this part of the market, if you want to compete against the big boys, you need big numbers, and I hope Aston can at least close the gap to Ferrari, given it is launching the DB11 3 years after the F12. Also, it isn't a good look when you have a brand new twin turbo V12 producing less power than the twin turbo V8 from Ferrari and Mclaren.
Anyway the engine output is till just a rumor and I will be very surprised if it isn't near the 700 hp mark.