> Are you saying that the only purpose of Target Diff/Vardiff setup for stratum is to identify how "often" miners respond to server?

What other reason would there be?  Every returned hash consumes bandwidth and CPU, on both ends.  If it didn't, there would be no reason to bump the difficulty.

> I'm not sure if this is what you are looking for, but there is actually a way to allow for user defined difficulty in stratum settings.

IIUC, the protocol allows the pool to define a minimum difficulty to the miner, and there is an out-of-band indication at some pools that the user can give to request a different minimum difficulty, but it's not actually part of the stratum protocol itself.  I am proposing that the protocol itself incorporate a way for a miner to always mine shares of a higher difficulty, and prove that he is doing so.