I'll disagree about ISMI springs and go farther in saying they're better than Wolff. They last longer, and suposedly of are more consistent weight than Wolff. I've also never had to trim a ISMI spring, even when running a shock buff(although two shock buffs and an 11# spring did cause problems).
I ran Wolff 17# recoil springs in my .45 for years with no problems. I am now using an ISMI 16# spring, and after maybe 1000 rounds am completely satisfied with its performance. I haven't checked the length or done anything to test it, other than keep shooting. They have a limited warrantee; I think they'll replace it for any reason within a year of purchase. I did have to cut a coil or two; fitting instructions are included.
The ISMI springs are superior for several reasons- they are engineered differently, are made from higher quality materials, and the manufacturing processes are more advanced. ISMI springs last longer and have a 1 year replacement warranty.