In addition to their seat kits, Seat Concepts now offers a complete bolt-on seat for the 2008+ Yamaha WR250R and WR250X. The complete seat consists of their foam and seat cover installed on a Spiral seat pan and is ready to install on your bike in a matter of seconds using the OEM mounting points (2 bolts). Fit and finish is top notch and the comfort level on these seats is great. The seats will work fine with the stock gas tank as well as the IMS aftermarket tanks.
I've been riding with Seat Concepts seats on all my bikes for the last few summers and highly recommend them. I run the low foam version on my WR250R and WR250X, so not only does it help me with the bikes tall seat height, but the seat is also much more comfortable than the OEM seat thanks to a wider seating surface and slightly softer foam.
NOTE: Seat Concepts still offers their seat kits (foam and cover only) if you want to save some money and just use your stock seat pan to install their foam and cover (see "Seat Kit" link below).