I actually have a platform that I hit off of and the main reason for this is a short throw projector that sits in front of the hitting area. So when I putt, the ball falls off the overhang platform over top of the projector. So in a matter of a foot after the contact, the ball hits the main floor and rolls where ever. So as far as the speed of turf to hit from, that is really no effect as the ball is picked up on the GC2 within about 4 or so inches of movement. So no matter how slow of a surface I putt on it would really make no change in the initial ball speed and if it did it would be such a low fraction of effect. Bigger effect is how smooth that putt rolls and if it jumps or wobbles off line in that first few inches of travel.
Bottom line is if you putt a ball about 4.2 mph (stimp speed), it would be hard to see or feel a change in putter (club) speed whether you putt on glass or low pile carpet. The ball speed is set so fast from the first movement of the ball that chances are it is still slightly in the air as the GC2 sets its initial speed and line. If you can hit the ball at that 4.2 mph or very close to that, you will see in game on a flat green the ball rolls to the stimp speed in feet. So 8 feet on 8 stimp and 14 feet on 14 stimp. I personally have never worried about my putt area and what real stimp that turf was but rather how straight the ball would roll.
On a side note one thing that would be a helpful test is if there was someone that had a real putting green and a real cup to putt to and a turf stimp speed that could be set the same in the game and do some test putts to see the speed a ball will still drop in the cup before it jumps over compared to what is in the game and also compare edges of the cup where lip outs would occur. I like where things are now but just wondering how close to real that it is. Possibly a hole cut in turf may react slightly different than would a real green cup cut into grass and dirt where the ball would tend to bounce off of.
Another test that would be helpful too is if the 4.2 mph ball speed rolls accurate to the stimp speed does it still roll accurate then if you are able to putt to longer length like 12 or more feet. Even if you are not sure how to calculate this if you could preform the test and then just tell us the stimp of putting green and then the ball speed to the 2nd decimal that was needed to roll the different distances even if they were measured in inches. This would be interesting info to have. I don't have enough sim space to have a putting area of more than 4 feet.