The i5m is archaic compared to the sounds and styles in the PA80. It will be a major Downgrade if you do that.
1/2 the polyphony (32)
older, dated samples
older, dated drum styles (although, some are very usable)

In general - the entire architecture of the unit has been udgraded since the "i" days. Why not concider getting a very small, weighted key controller to slide UNDER the PA80 and trigger it from that. Maybe the Ensonig is too big to seem appopriate. Fatar makes really skinny controllers that feel great.

Just a few options for ya.