I've had good luck with the brands you mentioned, I had a Guts tall seat on my DRZ400, and have an SDG
on my XR50. But Seat Concepts
seems to be the hot ticket these days, check them out.
As for dampers, I'm a big fan of Scotts
, but GPR is good as well.
The new (2012+) WR450s already have a power bomb
(expansion chamber) header of sorts stock. I'm not sure how much, if any, better an FMF
one would be. A Mega
bomb might show some improvement over stock, but personally I don't find the WR lacking for power or bottom end. YMMV.