I personally do not consider the recoil of .40S&W in P99 severe enough to consider a recoil reducer . In my book these devices are somewhat of a gimmick . I once shot my Sig P220 in .45ACP and my buddy's P220 with Harts recoil reducer in it side by side and did not notice much of a difference . More of a placebo effect .
I got the Sprinco recoil reducer for my P99 (9 mm) a few months ago and I must admit that I was not impressed. Although it's a well finished piece, it proved to be too long for the slide of my 9 mm (which is a bit shorter than the .40 slide), so I had to shorten it about 4 mm. Also encountered problems due to the edge of the forward part of the guide rod bouncing against the circumference of the bore (passage hole for the recoil spring guide rod) in the slide and leaving marks. I also solved this by rounding the edge of the rod but by doing so, the weight decreases and the dampening effect also decreases a little bit. After these modifications, it functioned satisfactory. I keep it for shooting 'hot' loads while I shoot moderate loads with the factory spring.
I got the Sprinco springs for both my 9mm and .40 P99. While it doesn't do much for the muzzle flip, it does appear to help with wear and tear on the wrist, when I'm shooting 200 rounds at the range. Both springs have operated without incident since I installed them.