I'm sure someone else will chime in with more 'knowhow'... . but if you don't change the fueling, then you won't be making any more power. You can even crank the heck out of the boost, but all you will be doing is burning the fuel more completely that was there before you went to twins... .
The spool must be interesting... . someone who has a setup will have to talk about that.
The actual horsepower that would be gained JUST by installing this setup probably wouldn't be much... .
The real difference is how much fuel you will be able to add before the EGT's get out of hand... . that's where you make the power. The twins setup is capable of a LOT of boost, so this equates to being able to run lots of fuel for more HP.
Matt - what do I know?!