I just recently got a set of Tune Skyline RRs. They're Stan's 340 rims. I was told at purchase that there were only certain tyres suitable for use with the 340s - and that they would not supply GP4000s, or Vittoria pretty much anything to go on them. I ended up with some Schwalbe Ultremos, and am reasonably happy. My LBS mentioned it as though it was reasonably well known, so I didn't think much of it.
Well... that is news to me. Can you get some clarification on what tires they *do* recommend.
Stan didn't forbid using GP4000... although he did mention that some German riders experienced GP4000 blowing off until they put on the two layers of 21mm tape. I caution against assuming too much just because some GP4000s have come off. It's a very popular tire, so it may not mean they are more prone. And I'm pretty sure all the top Conti tires have the same bead, so another Conti model would not be any better.
In fact I wonder if any kevlar beaded tires are safe... aren't the beads very similar? The Contis go on tight, but get loose after awhile. The 340 rim stretches the bead more than most. I just took one off the 340 rim and mounted it on an XR200, and it was very easy... too easy to put on. I pumped it up and the bead wasn't seating evenly... making the tire lumpy. A new tire fits tight on the XR200. It doesn't seem right for a bead to permanently elongate that much.