I still think the IRS is the way for the Mustang to go. I think the SRA is at the limits of its capabilities in the handling department, without major help from the aftermarket. Road racing mine, a watts link was vital for me to keep the rear planted firmly throughout corners and I think a proper IRS will be even better.
However, this is going to come with some shortfalls. Price will increase for every model of the new Mustang. That, or there will not be as many standard features on it like there is nowadays on 2013+'s. I think Ford will do a decent job with the IRS, but the reliability of it scares the hell out of me.
For me, I will give the S550 one whole model run before I get rid of my 2013. Plus having a ProCharger strapped to it right now is enough to keep me happy
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