Bill Repucci had a 290 on his RV-9 until the engine met an untimely death due to an unfortunate collision with a piece of ground equipment. To my knowledge Bill is replacing the 290 with a 320 because the scarcity of parts for the 290 makes the rebuild too expensive. (Chime in here, Bill!)
If it is fuel economy that drives you to the 290, I think that is false economy. While you may loose 10 knots of top end with the 290 you could throttle back a 320 to the same airspeed and have nearly identical fuel flow.
All of this is mute if you happen to have a 320 that you got for free. Then again, if that is the case maybe you could sell it to Bill for parts.