Those are not Roush seats. Those stock GT seats.
Depending on the year, Roush offered several variations of a more aggressively bolstered seat that the stock Ford shape. All are worth big money.
Prior to 2001, these were the Roush leather seat option, with the ROUSH name embossed into the corner patch.
These were 2001-2002:
2003 and some 2004 got this style (picture is of the Coddington Edition, but the Stage 3 got the same seat with ROUSH instead of Boyd's signature):
These were late 2004 options. (Pic happens to be the 440A option, standard just said ROUSH):
1998 Roush: M112'd Aluminum block 4.6L 2V 6 speed track car
1999 SVT: Mostly stockish
1970 Mach 1: Resto-modded monster
2000 Roush #5004 - World's FIRST M112'd V6 Mustang. Sold, but not forgotten.