Changing to a single motor set-up is certainly major surgery. (read Bryan's thread on converting a Hammer to 700 surface drive on offshore to get an idea of the work involved)

The boat will react quite different from the current two-motor set-up.

If it's going to be a lot faster, I can't say, as I have only converted single 550 motor boats to a 700 set-up.

The 14,4 V is the max for the two motor set-up, where as you can run a 700 motor on 12-14 cells.
If the boat runs free even 16 cells are possible.
Then it would definately be much faster (and a handful to control...).

Regards, Jan.