Hi! Bruce Gray here. If I may, please try backing out the trigger over-travel stop screw. It's possible that the hammer is being slowed by the safety sear on the way down due to inadequate over-travel. If you find that you cannot diagnose this issue, let me know and I'll have one of my men correct it for you as a favour to an HKPro member, if you'll send it in to us. There is no other inherent reason why the Match kit should be less reliable in this regard.
As for reduced mainsprings in general, we do fit somewhat reduced springs to many HK's as a matter of course in our comprehensive action packages, but NOT in absolute preference to factory mainsprings for carry and duty applications. We of course advise our clients to test with their ammo thoroughly before trusting such springs for carry, and we advocate use of factory springs for this use in any event. That only makes good common sense. We supply RP springs for range use, with that proviso that our clients can make that decision.
In practice, my USP's always fired reliably with 12X springs with all but the most challenging foreign ammo. For example, I was forced by circumstance to run some questionable PMP military ball at the World Championships in South Africa in 2002, and I had a couple light hits on the warmup range that scared me silly. (Fortunately my USP9F ran perfectly in the actual match and got me to 8th. I loved shooting for HK; my stuff always ran!) Thank you for reading and for supporting this great Forum. -Bruce