With tires in SP sometimes it is as much what sizes you can get and what it does to your gearing as much as anything else that you can think of.
the widest 14" tire you can get would be the 225/50/14 V710/hoosier currently.
Currently you are running the 225/50/15?
How often are you topping out second gear/shifting to third?
The 14" tire would be umm an inch smaller in diameter so your speed would be down about 2.5 MPH at 60 MPH.
according to my rough calculations assuming the P5 has the same gear ratios as the MSP your current tire (225/50/15) should get you to 60.98
MPH at 6500 RPM (not sure what your redline is) with moving down an inch you should be at 58.43 MPH at 6500 RPM in second gear. not sure how much time you spend in the range or if you are already shifting to 3rd on a lot of courses or not.
see the attachments for the graph version.(1st is 14"'s second is 15"'s)
Also I think 14" wheels are harder to find than either 13" or 15".