Can't give you a definitive answer specific to the IRS configuration but I think it really depends on how big you want to go, the wheel back spacing, and how far you're willing to go.

attachment.jpg

Above is a photo of my car that uses a solid rear axle. I can fit up to 31" diameter radials (radials don't grow) or 30" wrinkle wall M/T drag slicks under the bobbed rear fenders. If I really needed more, I could manage it. I mounted my body ~1" higher in the rear, use a parallel four-link that allows adjusting the rear ride height much more than the standard FFR suspension will support. If I had wanted the IRS and needed to run tall tires I'm sure I could make the modifications to run larger tires.

So, my take on all this is it just depends on what you want and how bad you want it.