-- Last Updated: Sep-30-12 3:13 PM EST --
I believe the rear of the seat pan only needs to be trimmed if moving the seat rearward in the composite boats. Not a problem in the RM models.
I moved my seat back about 1 inch. I'd consider this the "default" adjustment. After removing the seat (WS does have a video for this - Youtube, I think), you'll note there is enough room about 1 inch forward of the forward mounting hole in the seat pan, to make another hole. You also have to make a new hole 1 inch further forward for the rear hole. You end up with 2 sets of mounting holes on each side of the seat pan, and 4 machine screws still holding your seat in place.
edit 1: Here's the video. Note that this begins with the high seat back that comes with some (all?) Tsunamis, and which could probably be fitted to your Tempest, and which you MUST avoid, at least for a good while of working with your back band:
edit 2: I have noted that you would NOT be able to put in the high back, after all. The seat pan is molded differently. As the video shows, you can put a back band in a boat that came with a high back, but, you can't go the other way, unless you have a model with a seat pan designed for a high back.