I bought the Bushnell sight in the classifieds already modded. It's a good job too. I compare it to my Huang and Hurricane 552 sights, and it looks pretty convincing, although it's of a thinner metal than these other sights. Still pretty strong though.
And yeah, I guess there is an FN thread. Never thought of that. But still, the P90 is "special" and deserves its own thread.