Looking at that photo right there and seeing your seating position I'm going to make a recommendation that you take a professional driving class before you do any other modifications to the car. You'll be money ahead by investing in some training and learning some techniques (and why those techniques work) before trying to improve the car. ST class Mustangs are hard enough to drive well. Don't make it worse by not using the right driving tools.
Every man I've every met thinks he's an excellent driver and a fantastic lover.
Of course I don't have some formula to give you on how much it would be, not is there likely a way to quantify that even remotely accurately. That would be highly course/day/car dependant. 
OK, didn't mean to offend either. I honestly forgot you were on GC coilovers - I read a lot of interweb posts and I "mis-remembered" seeing Koni and assumed lowering springs. Sorry. I wasn't saying you have to buy 4150s or MCS singles or whatever - I know you probably won't - but that possibly you should upgrade from lowering springs and OEM style Konis. Which you have.
But there are still areas of the car to work on. Sure, rear UCA and rear adjustable bar are more things, but also: full lightweight exhaust, exhaust headers, lightweight racing seats (weight + control), front LCA poly bushings, and more camber. -3° is almost enough, but I'd shoot for more. A lower ride height helps get more camber at the same plate settings. And of course more tire...
While the STX winners were only on 245s, those were cars that were what, 600-800 pounds lighter? I assume they were FT86 or E36 cars. The same width tires on a much lither car = more mechanical grip. As the heaviest car in class you'd want to be the first one on wider tires, and can benefit the most from that. I think you are underselling how much this would help.
Its all good - do your thing.
) to make prepping the car to the max happen but wheels and tires are the priority right now. That is followed up very quickly by the rear diff. Those are the plans for the next year's autocross parts budget.
(had just the Koni Yellows and Steeda Sports)! The RX-8 he was referring to is decently prepped and could run with the Charlie Davis E36 and the Tsang brothers' E36 on a good day at the hands of a good driver. I think he was referring to my times compared to the RX-8 owner lolll but yes I agree in his hands with enough time in the car he can probably work some black magic (though at the school I managed to better his limited 2 runs in my car). But yes, Bryan's words to need to be taken with a grain of salt. I don't think there's a car out there that Bryan would summarily dismiss when it comes to autocross...but Mack Tsang also drove my car at a event in STX trim and called it a cow LOL. . 
Close. The turning point is when the LCA inclination at whatever ride height is perpendicular to the strut inclination at that same ride height. Lowering beyond that starts dialing camber change the wrong way. Whether any given McStrut car has enough suspension travel to actually reach the perpendicular condition "depends".
OKC is pretty far for me to drive for an autocross, but if you came down to DFW and I didn't have another race planned that day, I'd do it.
www.http://dfwautocross.com/ - We have 5+ clubs that autocross usually once a month, so there's always an autox going on here, pretty much 12 months out of the year (but January can be flaky). Some months I'm already booked racing 4 weekends in a row, but July and August are pretty light.
Sounds like you're getting serious over there.
Budget is pretty limited but these are modifications that are becoming more a necessity for longevity than performance enhancement, although they will also have that purpose.
