True you could change the shift time.. but then that would technically make the shifter 'unrealistic'
HOWEVER.. you have something going there with that.
A long time ago back in the days of the RSC forum, there was a thread.. I don't remember who started it (could've been me for all I know)..
It isn't anything new to see the driver in the game shift the gears, GPL had it, Gran Turismo has it, Forza has it.
The arguement for that solution however was.. what if you are using an H gate shifter, then it would technically make it unrealistic because the game would be shifting alone.. OR.. have the game know if you're using an H gate (by having keys set for each gear would be the easiest solution), so it wouldn't have that delay.
Weight penalty, yes I agree with you, but LFS has always been about trying to get the right mix, the FXO, RB4, XRT leveling battle has been going on since S1, and even today they aren't perfectly even cars, not because of how they react, but as simple as it comes down to weight and horsepower, yes vehicle dynamics makes it much more complicated, but keeping the playing field is not easy to balance.
Another old arguement will probably be brought up after this.. well if you're going to force everyone to have the same equipment, then.. why not force everyone to use the same everything including setup to? Again.. Leagues already do this, and they don't bother with public races were you can just get on, have some fun, get off.