Teague said a few months ago in his tech article that the IMCO was a faster lower than the sportmaster that you have.
I think what you have is better than a stock length lower, plus you already own this one. No real reason to change what you have.
I would want to take that spacer out to see if it is needed for how you boat. It may be that the previous owner wanted more of a water-sports boat than one that was setup for speed. The deeper the drive is the better it will be for slow speed stuff like dragging kids on a wakeboard & such.
It is possible that Velocity sets their motors high in the boat & the spacer & shortened lower was needed to get the correct drive height.