biggest slick i can put on a 15x8 rim without modifying the fenderwells? and the car is lowered 1"

 

Check the manufacturer's recommendations on wheel width vs. tire width. Hoosier will accept a width of 8-10" on their 28x11.5-15 QTP (28.0" diameter). I ran that tire on a 15x10" wheel with a 7.5" BS. It didn't hit on anything until trap speeds went over 125MPH. Then it started hitting on the front/metal edge of the wheel opening, and I had to move the axle back 1/4" to keep the tires from hitting.

The M/T ET Street 28x11.5-15 will also work on an 8" wide wheel, and they shouldn't be a problem because they are only 27.6" diameter. If you go to a true slick like the ET Drag, they will be a problem. I put a 28x10.5-15W ET Drag on the car, and the diameter was way larger than 28".... I ended up cutting about 1.5" off the leading edge of the rear bumper cover, and hammering the sheet metal under the cover.

Whether being lowered is a problem will depend on what the offset of the wheels is. Anything less than 5.5" offset, and the tire lip is going to come right down on the edge or tread of the tire. I've never had any problems with the fender lip, but my car sits 1/2" higher than stock because of the Eibach Drag Launch springs.

https://www.hoosiertire.com/Drtire.htm

http://www.mickeythompsontires.com/strip.html