with the car 2" lower on drop spindles (with angles staying the same), ...doesn't that makes the suspension is 2" lower? ...so, is not the instant center 2" lower?
Sorry, you're right there. The instant center location won't change relative to the car with the drop spindles. But, since the whole car is lower, the instant center will be exactly 2" lower as long as you leave the torsion bar adjuster settings alone. The caveat to that is, 2" is a lot of drop, even from factory height. If you don't use all of that 2" of drop, you'll raise the roll center when you compensate higher with your adjusters. So, if you drop the car 2" your roll center will be 2" lower. But, if you only go 1.5" lower, your roll center will not be 1.5" lower, it will be higher than that.
But, the instant center will be more than 2" lower on a car lowered using the torsion bar adjusters because of the change in location of the ball joints relative to the frame. That's actually why Peter (GMachineDartGT) went to drop spindles for a little bit. He was lowered to the point that the control arms were actually angled up slightly, so he was getting a negative roll center. I know he stopped using the drop spindles, I think it was due to rim clearance.
That's why the best geometry occurs with the LCA's about parallel to the ground.
As far as the suspension travel at that point, with the "button" style bump stops I have about 1.5" from the frame to the bumps on my Duster right now. The LCA's are pretty close to parallel to the ground, and I have a little under 4" to my header flanges. With 1.12" bars I don't suspect I will ever hit the bumpstops. I actually plan on dropping the car slightly to put the header flanges at about 3.5" of the ground. If you take a look at the picture I linked earlier showing autoxcuda's car, you see he has about 1.5" from the frame to the LCA (no bumpstop) in that picture. That was with 1" bars, and he said he never hit the frame. With the buttons installed at that height you'd have a little over 1.25" of clearance from the bump to the frame.
I have my Challenger just as low now, lowered with the torsion bar adjusters, as I did when I had it lowered with 2" drop spindles. Again, that's with 1.12" torsion bars. I haven't had any issues with hitting the bumpstops, and I actually drive the Challenger semi-frequently on gravel/dirt roads that have serious pothole and "washboard" issues.
The trick to losing the suspension travel is going with bigger torsion bars. You just don't need that much travel anymore. And since I think that the right spring rate for these cars is in the 250-300 lb/in (1.04" to 1.12") range for most street/autox/roadrace applications, you can lower the car that much without worrying about the loss of travel to the frame, since your actual travel will still be less than that.