I have a question on the difference between 2.1 and 1.9.
Speaking purely for FH as an example, you have a good technique down. Opening Loop, Close to table loop, mid distance loop, loop to loop.
How would the MM of sponge change the outcome of the exact same shot of each of the above?
I've used the same rubber in 2.1 on FH and 1.9 on BH for a long time. There is a difference, but it's a slight one, so subtle even that an inadvertent twiddle might go unnoticed.
The 2.1 in my experience has a
slightly higher dwell,
slightly higher throw,
slightly higher speed (and perhaps a
slightly flatter arc, but if so that's
very, very slightly so) when spinning, and a
slightly more error-prone direct hit and
slightly less oomph on aggressive punch blocks. I suspect the way you're hitting it makes a larger difference than the sponge thickness.
That being said, I now use 1.9 on FH and feel more unimpeded,
slightly more confident in making full strokes, unimpeded.