737
Transponder ALWAYS transmits levels based on 1013.25. What you have set on your altimeter has nothing to do with it.
(ATC radar displays convert to altitude below Transition Altitude by having the QNH input into the radar processing).
So - with all Transponders emitting the data based on the same datum, there will be no discrepancy in the TCAS information shown no matter what is set on your altimeter. TCAS WILL NOT be affected by setting QNH early, when descending to an altitude (in fact, it's better during low pressure to do so - otherwise you risk a level bust in the descent).