I installed a set of Eibach's in mine yesterday. I need to get it aligned (the rear camber is quite visibly off now), so I haven't really tested it too much yet, but the 10 or so miles I've put on it don't feel any different than it did with the stock LX springs. I didn't change the shocks, which are a set of Tokico Blues that I put on almost 4 years and 70,000 miles ago. I had to replace the upper front control arms, because the upper left ball joint boot was shredded, and the joint itself was trashed. I also put in new rear spring rubbers.
The only clearance issue I can see is the exhaust. That, and the little bracket that spans the driveshaft tunnel. I have a dynomax catback on mine, and the main pipe just forward of the diferential is the lowest hanging part. The underbody just behind the front fender is just above the top of a Pepsi can now. Everything else looks like it will go up with the wheels when crossing speed bumps, so high centering something is my only real concern now. The had pipe and third cat hang below the tranny pan deep sump, and the fuel lines/filter are protected by the "frame" rail.
My impression is that this is how Ford should have done it in the first place. Not too much, but just right. The ride is still great, the look isn't attention getting, the tires fit the wheel arches. It doesn't "bounce" or bottom out, like my Fiero (with Suspension Techniques springs Monroe shocks/struts, and poly control arm bushings) does.
If you want to see a lowered t-bird without a lot of "bling", I took some pics today, and I should be able to e-mail jpegs to anyone interested in a few days, after I get the roll developed.
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...