I have two IIc with both lenses in 2,8, and the Heligon is sharper than the Xenon, but it could be due to sample variation. And the difference between the Curtar/Longar and Heligon-C 35/80 is the same, the Heligons are sharper. I suspect that the rear cell of the Xenon isn't the best on my camera. Maybe yours is better.
Stop down a stop or two, and I think you will find that the Curtar and Longar will perform about the same as the 50mm. The 35mm will vingette a little in the corners, but it's not that bad.
I haven't used them for "real" photography yet, but I have some quick test shots on my flickr page. But remember that your Xenon may be better than mine: http:[email protected]/