will a slip on exhaust can from a k7 fit a k8 without altering the cat box in other words what will fit my k8?

Without altering the cat, it cant be done. To be able to do that u would have to cut off the cat, install a mid pipe, then basically attach the slip on. Not sure if u would have to remap it tho, probably will. Plus u would be voiding out ur manufactures warranty. To answer ur question, only 08 slip ons will fit the 08 gsxr.