INXS,
A single 25" shaft outboard on a banana is the same as twin 20" shafts due to the deep vee shape of the hull. In both set ups the prop shaft is about the same distance from the bottom of the hull.
As for outboard set ups, here are some expectations. This info is based on no less than 8-10 different banana's with twin outboard set ups both on the transom and on jack plates.
1. Twin 20" shaft 200hp "stock" outboards mounted on the transom running 24p chopper props. (65 mph).
2. Twin 20" shaft 200hp "stock" outboards mounted on 5-6" manual jack plates with counter rotating lowers, (turning out) and nose cones with low water pick ups running 26p chopper props. (70 mph)
3. Twin 20" shaft 235-240hp outboards mounted on the transom running 24p chopper pros. (70 mph).
4. Twin 20" shaft 235-240 hp outboards outboards mounted on 5-6" manual jack plates with counter rotating lowers, (turning out) and nose cones with low water pick ups running 26p chopper props (75mph).
5. There was once a stock banana with twin Mod VP's (280hp) that were mounted to 5" manual jack plates with counter rotating lowers (turning in) and nose cones with low water pick ups running 26p chopper props (claimed 80mph).
(note) Switching lowers so they turn in may give you a 1 mph gain, but you forfeit performance when docking and it makes things a bit more unstable at speed on some boats. The cool thing is it gives you a little rooster tail from the moment you add power to when you get on plane, then it goes away.
6. B 73 "the second one" ran on the transom with 280 hp Mercurys. I have not talked to Mike Snow to confirm a top speed, but the boat was advertised as 85mph last time it was for sale a few years ago. (I think it's possible as the hull was a balsa cored race lay up as Spoiled Banana was. Spoiled Banana did 75mph with stock 200hp Mercurys running on 5" manual jack plates.
Hope this helps...
Chris