All of my GP100s (and I have...6) have the Wilson 10lb mainspring. Some have the stock trigger return spring and some a lighter trigger return spring. I won Master class at IDPA Nationals in 2014 using a 357 Mag GP100 with a 10lb mainspring and 8lb trigger return spring; this year I've got 2k+ rounds on a 10mm GP100 using that same spring combo. It's pretty tight.