Hi Jeff, thanks for reaching out to us, although in this case "us" is the whole forum apparently. Love all the support fellas!!
18x11" ET52 is a great square setup for track use, but to be honest the street tire selection is a little lacking when compared to 19x11" options. Now, if you have found some 305/35-18 tires that you like, then 18" wheels are the way to go due to reduced costs and weight when compared to their 19" counterparts. The Toyo R888R in 305/35-18 would be a good option for someone who primarily tracks the car but does drive it on the street once in a while. The Hankook RS-4 in 295/40-18 is another one of my favorites for HPDE/light street use. They won't have the ultimate grip that the R888R can offer, but they will be much more consistent throughout a 20 minute session and last MUCH longer as well. The value from the RS-4 is really hard to beat in time-trial events.
If you are doing time trial events and need a ringer 200tw tire, I would use the 315/30-18 BFG Rival S. Similar grip to the RE71R, but in a useful size for an 18x11" wheel.
My current running wheel and tire set up are the Project 19 inch with the RE71R. In the garage rafters are another 2 Project 19 inch wheels and a set of Sport Cup 2s that might have a single track day remaining in them. The Sport Cup 2s are 305/30-19.
When I mentioned going to a second set, the thought was really just purchasing two mode Project wheels and mounting up the Sport Cup 2s for COTA.
I like the idea of the 18x11 for lighter weight and a little less hot wheels appearance. The plan would still be two sets of wheels and tires, but, initially it would have to be a single set with rubber that could street and road course.
I do have a trailer for towing the Mustang to the track...
Thanks for all of your inputs. I will consider the options and decide my course of action. I figure the sale of all my existing wheels and tires could closely fund my 18x11 wheels and tire purchase.