I put a Kenny Brown front STB in mine, and I think it helped. Keep in mind that's only by the seat-o-the-pants meter. With the X-brace behind the seat (mine doesn't fold down), I can't see where a rear brace would do much, but unlike me, Rich actually installed one, so I'd take his word. He's been around here for a while, too.
I'd either get an 02 GT MAF and intake tube and ditch the MAC CAI (looks like you already have a custom EEC program), or put a dimple in the MAC so it fits. I shimmed my STB with washers under the front bolts, to roll it upwards a little, so it would better clear the coil packs and plug wires.
97 LX, 4.6, Dynomax catback, Mustang MAF/intake tube and Aviator throttle body, Xcalibrator2 by Lonnie at BOC, 180 t-stat, Super Coupe wheels with 235/60/16's, Kenny Brown strut tower brace, Tokico Blues, Eibach Pro-kit, some strategically placed MN12 Performance polyurethane pieces, '02 Grand Marquis transmission
Submariners do it deeper...