I have a notch with the MM Koni coil overs all around and the IRS in the rear. I love it but went heavier on the spring rates and having the IRS in the rear did influence Jack's thoughts on spring rate somewhat for the front but in your case just order the right springs later like he suggested. The biggest problem with the IRS is the wheel choices are very limited given you need no more than a 9" wide wheel and 275 tire to tuck in the wheel well but you'll want a 6.75" BS on that 9" wide wheel.
All that said I still don't regret going IRS on my car at all, but I would say you should replace all the control arm bushings on it with AT LEAST poly bushings before you install it.
1992 5.0 5sp coupe, ported GT-40 intake, TFS 170 heads, Scorpion 1.72 RR, TFS1 Cam, BBK longtubes, BBK H-pipe, MSPNP2 ECU, Full Koni MM coilovers, SVE S1 seats, Moda R6 Wheels, full Cobra Brakes, 2004 Cobra IRS, MM SFC's,