"is a glider designed to the "16G cockpit" standard heavier than one designed with a rescue parachute (and including the parachute weight)?"

A designer would give you a precise answer but if you consider that you trade your back parachute against a rescue system, the delta is probably no more than 3kg (11kg - 8kg).
This is likely much less than the additional weight required by latest EASA regs on cockpit integrity.
The problem is that if you certify under EASA, you don't have the choice to go one way or the other. Therefore if you want the rescue system, weight goes up.