It is lowered on GT2 coilovers but even when it wasn't lowered as much it still did this.

 

No high speed vibration that is considerably noticable is normal , but I would recommend just getting your car properly adjusted ride height wise and alignment wise because that alone will get rid of a lot of what ifs?.., and I dont know if youve ever had your wheels checked because over time some roads can really change the dynamics of any wheel.

 

I had a 996 turbo s with stock suspension a while ago I was working on and he had a high speed vibration throughout the car all of the time (110mph>), I swaped spoilers and bumpers and I even messed with tire pressures, etc.... but it ended up bieng his rear tires after I swaped them out with another turbo rear tire/wheel set.I hope that can help your issue.
Regards..

 

Thanks for the help as I need ideas. Ride height is perfect-left to right, alignment is great too as Saturday.

-1 camber up front .02-.03 toe on both front total at .05. Caster is 8.1 and 8.2 up front.

Rear camber is -1 or -1.5, toe is .10 each side, with .20 toe total. Anyone in South FL have a set of 19's I can try for a 996 turbo, lol? Brakes won't clear 18's.

 

UPDATE: If anyone cares, lol. I found the left fender liner trim was loose and I fixed it, it helped a lot!

Then I remembered that a piece of the fender lining was torn off on both sides from the previous owner, I'm thnking that maybe the wind might be messing up the front end...

Anyone know where can buy a couple fender lining piece? Anyone do a GT2 conversion and have stock 996 turbo pieces left over?

 

I'm running 19's with the Toyos I don't have your issue. You can try my wheels/tires If you want to eliminate the possibility, I'm in NE Broward.