I really have to wonder what it would do to the balance of the car, especially if you shoehorned the 2.3 in there.
Mazda had a shop build them a Mazda 2 with a Mazdaspeed 3 2.3 for SEMA a few years back, and apparently it was absolutely awful to drive. They couldn't keep temps under control, the car was totally unbalanced in the corners, and wheelspin and torque steer were out of control.
Now, what I'd love to see is our 1.6 in a Mazda2, that'd be an absolute riot, since the 2 is almost 500lb lighter than the FiST.