Since HiRpm posted he did not want to buy expensive things twice, that was my main motivation to suggest certain tires and rims, and the shorter tires ( than stock diameter) are being used almost across the board by the faster drivers ( track and autocross). Over the years I learned the hard way, getting what I thought was the hot set up and then getting to the events and finding I was behind the times. Many have had this discussion at the Mustang Roundup and once folks see what others are running, the next year the bulk of those competitive spirits, end up with more rubber and a smaller overall diameter. Heck, I am running a 315/30/18 on advice from AZBoss and since he won the Championship in his Class ( NASA AZ Region), I give him credit for helping me do the same in my Region, last year. I would probably have gone with a different tire if completely on my own.
So, my advice is based on Hi not wanting to spend extra bucks if he gets into Autocrossing , and of course he will need to determine his Class also , since he may have to go with a narrower tire or rim size. There are those on here that can help him with that ( DaveW, Autoeux, me, etc. ) but I will constantly remind those of one noted authority , Blacksheep 1, who has often mentioned on the Forum that this machine needs more tire........................that is probably the best source of advice for anyone, considering his record with Phoenix Racing and others!!