In my humble opinion and based on my very limited experience I think on a non J I could see going to the LoPresti cowl, MAYBE if it was 2-3K max and then maybe ??????? But absolutely positively NOT for a J! Not for 13 -15K $ !!! The J cowl is refined to maximum aerodynamics. I don't see spending this kind of money for a questionable 1-2 mph if that.
I would rather remove the RAM (SERVICE INSTRUCTION M20-93) hardware, and take the J cowl to an autobody shop. You can have a beautiful cowl perfect and painted for under 1 grand!! At least this is what I was quoted. I plan to do this at next annual. For more of an cosmetic improvement remove the landing light also. I plan to place landing/taxi lights in the wings. Total cost parts and labor ~$2600. (2 wing mounted light fixtures, labor to install them, remove RAM and refinish and paint cowl).