Well I know he grenaded the IRS at the track and swapped in a solid from a 99 gt with the same borla exhaust so I don't think being IRS matters but I'll double check with him. 99 gt's and 98 gt's can swap exhausts right?
IRS exhaust runs under the IRS, not like a solid axle where the tailpipes run up and over top of the axle then exit at the rear of the car. If he swapped in a solid rear and the tailpipes go up over the axle, the exhaust should fit.