Lowering Spring help??

[Helo00]

GMPP lowering springs would reduce the front ride height by .9 inches and rear ride height by .7 inches on my 2000 Pontiac GTP. Winter season is here and my 18" wheels are taking a break but I was wondering if I would still be able to put these springs in my car and not cause rubbing problems with my custom wheels. The wheel dimensions are:

Specification:Stock-225/60-16x6.5/Custom-245/45-18x7.5
Sidewall: -5.3in/ 4.3in
Radius: -13.3in/ 13.3in
Diameter: -26.6in/ 26.7in
Circumference: -83.7in/ 83.8in
Revs/Mile: -757/ 756
Difference: -0.0%/ 0.2%

I would really like some of that space between the tire and the fender to disappear, however I don't want to get new tires because the Nitto 555's on my custom wheels are brand new. Can any of the Pontiac/GTP masters give me advise on this? It would be greatly appreciated.