I heard somewhere that you need to change the suspension if you go with 20" wheels. Is that true?

 

i doubt it, alot of people are running 20's on this car and for most of them that is the only mod they have

 

how much difference in weight will the new tires and rims be from the old tires and rims? If they weigh a lot more, maybe some better/hd shocks and struts might be in order

 

They are Saleen's that I would buy used from some guy. If they are replicas, would they be lighter?

 

I don't think the wheels will weigh that much more than the stock 18's, as far as suspension no changes will be necessary just for changing the wheels, however; if your like me, the change in profile of the tires make the fender to tire gap seem larger, therefore; lowering the car would probably be more visually appealing. I haven't had any issues so far --2,000 miles.

 

I had my 06'GT lowered with FRPP handling pack.I ran the twenty inch Raceline GT's.It was a very swift corner cutter but lots of road thump and fedback.The look was appealing but not worth the constant watch out for pot holes.
If you lower your car,consider the Steeda Sports,IMO they have a spring rate very close to stock.I run them and love the ride on my 07'GT.