I have not used XCR Goretex in a bivy, but in my Arc'Teryx Sirrus Jacket, the XCR variation of Goretex has proven to give the best breathability of any waterproof/breathable lamanate I have used.
For frame of reference, I have used traditional goretex, hyvent (North Face), genesis (Sierra Designs), Precip and Precip Plus (Marmot), Epic (Integral Designs), and Elements (REI) brand membranes, so I have a decent frame of reference for comparison. I have heard great things about eVent, but have only used it in a short gaiter, which doesn't make for a fair comparison.
Also, no matter how breathable the material is, venting is the other factor which will have a huge effect on overall breathability in a system. If the bivy does not offer a zipper for some sort of venting from time to time, you will still most likely experience some noticeable condensation. I use a non-zippered bag cover occasionally that usually retains a bit of moisture despite being a "breathable" fabric.