I can’t make a direct comparison between the CORE and X5 because I shot the CORE when I first started USPSA and I was shooting the cheap 115 gn FMJ. I sold it because I went to Limited. I then went to CO and that’s when I first tried the X5. Can I tell a difference between the standard X5 and the Legion? Yes. Nothing dramatic but there is less recoil. Does it result in better match results? Possibly, but most likely very minimal when shooting 125 gn at 135ish power factor ammo.
I know most USPSA competitors don’t shoot the M&P much anymore but at least IMHO the best gun is the one that allows you to get a consistent “USPSA” grip (I.e. high support hand grip with plenty of real estate for the support hand palm). My results were the accuracy and reliability were very similar as I had them setup.
DISCLAIMER: I am not very sensitive to “good” or “bad” triggers. As long as the weight is below 4-5 lbs for USPSA style shooting, creep and grit don’t make much difference to me because 90% of the time me I am pulling the trigger at at least 80% of my max speed. To me, all striker triggers are only OK. DA/SA (CZ/Tanfo) are much nicer triggers (except the first shot).