Like the others have said, get the 710s on 17x9s and it'll really help. That's the combo I'm running on my car, and it's about as competitive as a Mustang can get in SM. For reference: Last weekend I ran EM and finished 2nd out of 6, and beat the car/driver that finished 4th in EM at the 2004 solo nationals. Now I think he had a bad event, but it isn't the first time I've beaten him. A rotary turbo Lotus 7 finished 1st & 4th. BMW M3s were 5th & 6th.
Coil overs would help, but don't bother til you figure out your rear suspension. I've found you get a better ride with 400 lb coil overs than you do with 850 lb conventional springs due to the reduced vertical load & friction in the a-arm bushings.
I'm not convinced the 3-link is legal, unless you successfully argue it as a traction bar. While it bolts to the original pivot points, it relocates the pivot point on the chassis end. I'm running a 5-link, which has been protested and upheld in ESP, so is therefore legal in SM.
Besides the 5-link, I'm running SN95 length, 3/4" forward offset delrin bushed front control arms, X-2 balljoints, CC plates, Koni doubles, 500 lb coil overs up front, 500 lb conventional springs rear, urethane bushed rear control arms with weight jacks, SFCs, and stock swaybars with rod ends for end links up front. Cobra brakes F+R with adjustable bias. Got about 400 hp + tq, and weigh 2980 with a full tank with 55% up front. Sounds cheap, right?
In the DC area, our SM class sped up so much over the past couple years, the car that dominated in 03 can't trophy in what is usually a 20+ car class. But like you said, competition is more interesting than winning CP by several seconds.
If you want to go CP, skip the 710s, and get some 16x12s with 25x12 Hoosier slicks. Be prepared to cut your fenders...