I think the ITR Mustang can be extremely competitive in R. My research shows that it'll make class leading power, can get within 75 lbs or so of minimum weight with a lot of attention to detail, and since they are the same as ITS Mustangs with respect to chassis, they handle well.
A lot of attention needs to be paid to the suspension setup and particularly the rear suspension. Well, the front too, but the rear requires more parts and work with a tri-link and panhard rod.
Frankly, I feel the ITR Mustang has more potential than the ITS Mustang. I built an ITS Mustang because I wanted to race with my friends, but if that were not a constraint I'd build an ITR version.