A fellow musician here ... I can speak from experience, and tell you that the Sennheiser MX W1, or any of the Sennheiser wireless headphones, will not work for you. Anyone with a set of those can test the lag by simply turning up the speakers on your source (TV, stereo, whatever) and listen to both at the same time ... you can hear the delay, and professional musicians can not tolerate that. I'm thinking of trying the "Sleek Audio" ones, as I've heard from so called "audiophiles" that the latency is better, but they use the same "Kleer Wireless" system that the Sennheisers use, so I don't know. The only advise I can give you is, the Shure PSM200 system works with no latency, and is usable on stage (I use it), but I'm very unhappy with the compression (dynamic range) ... it sounds horrible, but it's better than hearing your own voice come back to you a fraction of a second later. I use "Line 6" wireless guitar systems, and they are incredible ... no latency, no compression, no difference between the wireless and my favorite cable, but unfortunately, they don't make in ear monitors yet.
... addendum: In my research, I just found this on another forum, no personal experience, but it sounds like anything with Kleer Wireless is not going to work ...
"I just got a set of Sleek Audio sa6 with the Kleer wireless system. The Kleer system is billed as low latency, but I guess that's a relative term as I'm getting at least 200ms of delay. That's much faster than the bluetooth sets, and the bluetooth sounds terrible - think 32kbps mp3"