I removed my resistor a few months ago, but was not putting in Premium always.
I found a station where I can get Premium for $1.36 per gallon, so I now buy nothing else. It now seems to have a little more zip in the medium to high RPM range.
It is possible, that once the knock sensor detects knock, it "learns" to keep the timing retarded. After awhile of not detecting knock it starts advancing maximum timing. I don't know if this is the case, but I know some other systems work similarly.