I put a sam James cowl on and found a 25 knot increase in speed. Also stall was down 10 knots. ............
Only kidding, did the exercise myself and found there could be a few knots in it depending on how well the installation is done. At the end of the day I opted for the standard cowl ,based on looks and ease of fitment.
One thing I have found is that the RV flies very nicely at low power settings giving about 7GPH and 135knots. I often putter around at these speeds.
I doubt the SJ cowl would make much difference at these lower speeds.
Sorry about not giving a proper answer, just thought my experience could be of some help with your decision.
xp360, CS, All Glass cockpit