I believe from what I read the stock recoil is 11lb. I ordered the 15lb because apparently due to the light weight of the frame, it has a bit more felt recoil than other 9mm mak pistols. I figured since I was already replacing the hammer spring, I might as well replace the recoil spring as well.
Be careful here (and this is why I asked). The recoil spring resists the movement of the slide as it travels rearward. If you use too heavy of a spring, your slide may not slide to the rear as far or as fast as it should. You can sometimes get ejection problems because the empty shells aren't getting kicked out of the port fast enough. You can get stovepipes or other ejection problems.
Too, heavier recoil springs tend to INCREASE felt recoil, at least in my opinion. "Recoil" is highly subjective to be sure, but a heavier spring will slow the speed of the slide down some. This means that it takes a longer period of time for it to travel rearward and then return fully to battery...so it's
moving for a longer period of time between shots. When I replaced the 18# spring in my CZ with a 13# spring, the gun really felt softer to shoot. Yes, the ammunition does kick the slide back faster, but it also returns to battery faster, and I'm back on target quicker. Again, "felt recoil" is very subjective...I wouldn't necessarily expect a heavier spring to make the gun feel more controllable, or softer to shoot, or whatever your metric is. It sometimes works the opposite of what you expect.
To check how well-suited your recoil spring is for your ammunition, check your ejection patterns when you shoot. Ideally, you want to have all of your spent brass landing in a circle behind you and to your right, about 3-5 feet. If they just "hump" out of the gun and fall near your feet, then your recoil spring is probably too tight for the load you're shooting...they just "hump" out because the slide isn't sliding rearward fast enough and the brass doesn't hit the ejector with enough speed. On the other hand, if they fly rearward 10 feet, then your recoil spring is probably too light for the load you're shooting, and the slide's traveling back faster than intended and the ejector really kicks the brass out. Aim for a circle, no pun intended, about 3-5 feet over your right shoulder.