Grip level is better than the R1, but I can't say a whole lot else yet..
I bought a set of takeoffs a few months ago and stuck them on the car right before the COTA race.
Had some weird handling issues and was scratching my head when the local BFG guy asked me why I had 3 R1S on the car and one R1 on the right rear. Hmmm might explain why the car was only loose in left turns!

Anyway, I switched back to another full set of R1 and the grip level was less, but not by a huge amount.

Laps times between the R1 and purple crack R6 are 'only' about 0.5sec. The R6 feel much more stable and have more linear breakaway in turns, but the R1 is faster on the straights- I'm guessing due to lower rolling resistance.

Not exactly the answer you were looking for, but yes- the R1S has a noticeable- not not dramatic- increase in grip.