On normal conditions what would be a safe final approch speed ?
Well.... in normal conditions, the safe final approach speed for a STOL kit equipped plane will be the same for that plane with an un-modified wing. But, that's not what you wanted to read. You'll have to refer to the Flight Manual Supplement for the RSTOL 337 for the speed. I've flight tested one, and done lots of testing on the slow side, but I'm not certain enough of the speeds I was flying to post them here as "data". You can fly a STOL equipped, particularly Robertson STOL much more slowly than the original.... But.....
Routine operations in these slower speed ranges are unwise, and the wording of the Robertson FMS suggests this. At those slow speeds, within a few hundred feet of the ground (either takeoff or landing), an engine failure - even in a 337, has a good chance of being unrecoverable to a safe landing. The plane will fly more slowly with power, but not without. A sudden power loss will instantly cause the need to accelerate to glide speed, which will consume more altitude than you have. An un-flared crash will be the result. Though this concept is not so well understood for fixed wing flying, it is very well known for helicopters. Search the "height velocity" or "Avoid curve" for a helicopter, and you'll get the idea. The fact that this vital information is not published for airplanes does not mean it does not apply! Flying a STOL equipped plane at speeds slower than Vy, is about the same risk as flying a regular twin around slower than Vmca - loose and engine and you're probably not recovering.
The added factor with the Roberston STOL kit, differing from Horton and Sportsman is that the drooping ailerons keep you hanging on so well at slow speeds, it's deceiving. You can develop a higher than expected rate of descent with power on a slow final, and the plane feels solid, and it is, BUT only more power will arrest the descent, you have no inertial energy in reserve. I've done lots of flight testing and advanced training in STOL and RSTOL 180/185/206/210/337 and this is the prime thing I train new pilots. The fact that the plane is capable does not make it wise. 'Same thing about those silly steep climb outs seen at "STOL" competitions - totally un-necessarily unsafe! The plane is off the ground - stay low, build up a safe speed, then clear the obstacle, you don't need to be 200' above the obstacle, hanging off the prop slower than Vx!
Find an RSTOL Flight Manual Supplement, and read it through, then note the carefully chosen wording about "advanced operations" and "austere conditions", and you'll get that Robertson knows that they've created modified planes so capable that only tremendous pilot wisdom will assure the greatest safety, with a reserve of energy maintained in case of an engine failure....