Some developments on my side:
The BIOS my card has, is, apart from the clock frequencies, exactly the same as the club3d 5900NU bios. I compared the two, and I only seem to have a different 5900nu version.
Looking at the thread "basic 59xxx bios changes" seems to comfirm it as well if I look at the 5900 bios part. The bios has a memorytiming of a 5900nu, but a memorylatency of a 5900u / 5950u. :P
Memory doesn't do 850, so I bet it has been relabeled to a 5900xt and sold as such ... still does 820 though, but that doesn't change the fact the core hardly goes beyond 400 ...
Looking at the BIOS I see I can increase the gpu voltage when in 3d ...
not without a risk ofcourse, but as long as I keep an eye on the temperature I should be fine, right? Alright, I'm off to try that out now.
Setting the core voltage at 1.5 gave me a nice bump in speed ... and temperature, in the mid 70s now in 3D, so kind of worried about that. Nevertheless a speedincrease: 450/820 atm. Memory doesn't go higher, and even though the core might, I do not feel like trying that untill I got me some better cooling.