I say go run with the "imports" Sunday morning and have a blast. Since you are somewhat new to your build and vice a versa, you will be racing yourself and the road more than the other cars around you. Once you become a "Stielow behind the wheel", then you can show the import boyz how it's really done. Real cars have rock chips, but see if you can keep from eating grass, well, you say
canyon which raises the stakes MUCH higher - your pilots license doesn't work in a car (DOH)
I did some Corvette canyon carving a few years ago in the mountains of Arizona and found road dust and sand to "make my heart skip a beat and also grease my seat", so let some drivers run ahead of you to see if they happen across anything before you do as an early warning. Always remember to ease into all of your maneuvers so as not to upset your chassis / traction unpredictably - brakes, steering, throttle... You may have to pull over and change your brake proportioning setting a bit too, as hot brakes in a canyon run will be completely different than most other "normal circumstances". Hmmm, sorry about rambling on about how to drive - you are just going out to have fun! Those high dollar "imports" don't want to hurt their cars either, so I'm sure you will hang with them just fine.
As far as 305 versus 315 tire tread width? 10 little millimeters? Not worth the trouble. Many don't know this, but as your tires wear, the contact patch width widens considerably. By the time you are halfway through your rear tires, they will probably measure out at 315 +/- anyway. Have you ever heard of certain class racers purposely sending their tires out to be shaved? Yep - they carefully cut more than half the tread off of a perfectly brand new tire to reduce "tread squirm", lighten up the tires for less unsprung weight, and also makes the tread area measurably wider (while still staying within the rules for their class). In your case, a few good burn outs can do this too, but spray some PAM no stick cooking oil on your rear quarter panels first and have the video camera rollin' while those dollar bills literally go up in smoke
