A few things (and I think it's relevent);
1. The info dipslayed to you, and the info transmitted, are not neccessarily the same thing (have seen this several times).
2. The integrity of TCAS relies on the integrity of the transponder, and the manner of determining the intregrity of the transponder is reliant on correct altimeter setting. So, I disagree, and think that altimeter setting can affect TCAS.
Make sense?
3. If clearance instructions, hence separation, hence whether you will experience a TCAS event, rely on correct altimeter setting, then the two (three) are permanently tied.