Are STOL kits that use leading edge slats available for 'typical' GA aircraft? Has anyone tried fitting leading edge slats to any of the 'typical' general aviation aircraft? If so, are the benefits worthwhile? I know they're available for the Piper Super Cub, but I haven't been able to find any for tricycle gear aircraft like a Cessna 182 or Piper Comanche. I know that having a  high wing or low wing will also be a factor.

The Helio Courier, a well known STOL aircraft, has a speed range from 25 knots to 147 knots. I assume that other aircraft may benefit for using leading edge slats or other devices to lower the stall speed to 25 knots or so, leading to slower, safer landings and emergency landings. All the advantages of the Helio, but in a different form that some people may prefer, for example the low wing aircraft configuration. 

The only disadvantages I can think of are the limits of the tricycle landing gear in achieving a high enough angle of attack when landing due to the possibility of tail strikes, and the better protection from debris
in soft field takeoffs and landings. 

And what would happen if someone fixed a Helio wing on a Piper Commanche, assuming cost no object?
Lowering stall speed is not a benefit in itself, it's only a benefit if it you need it. There are STOL kits for many airplanes, you don't see them much on most airplanes because there's no real benefits in normal flying and they cost a fair chunk of money to install. STOL kits only get you so far, adding a STOL kit onto an airplane with a 50 kt stall speed will not give you a 25kt stall speed, it's a lot less than that. If you have a bush airplane, fly into very short fields or in high density altitudes then a STOL kit can give you extra capabilities. If you don't fly into very short fields then a STOL kit is extra weight and costs for capabilities you won't use. 

It's also not as simple as simply sticking some extra bits on the front of the wing, there's many considerations. For one, if you drop an airplane's stall speed substantially then you will run into control authority issues at slow speed. You will have to beef up the ailerons, rudder and elevator to compensate, which is a substantial re-design, adding costs. The most popular STOL kits are for airplanes which already are good STOL aircraft to begin with and have adequate control authority.