My 182 P has a Horton STOL and VGs approaches at 55 to 65mph IAS depending on the weight.
How much will it lower the stall speed with wing extensions?

I live in mountain terrain, and 3 months of the year lots of wind.
Does the aileron authority is lost quite a bit?

What else can you guys tell me about this mod?

I fly a 180D (work bird) with Sportsman STOL, VG's, Wing X extentions. The ailerons are pretty sluggish at all speeds. The 185 I fly (also 135) has no STOL anything on it and there is quite a difference. My own '73 182P, no STOL mods on it either and it's also a far cry from the 180 with extended wings. You'll notice a difference for sure...both at cruise and at low speed. No more wagging wings to say hello to spectators on the ground.

I have the extension on my 182P. I noticed very little difference in aileron effectiveness before and after the conversion. My plane is used almost exclusively on floats. Takeoff difference is substantially improved on floats, they seem to allow you to takeoff with less nose up pitch, which on floats means less drag on the heels. There was no speed penalty on mine. I have no real world numbers for stall speed differences. I can definitely say it rotates and flys off the water at a significantly lower speed with the extensions. At least for float flying, I feel this is a highly desirable mod.