NeverSellingMyMustang

My Michelin 4S's are doing great but I'm doing planning for my switch to the Firestone Indy 500 when they are worn. My Mustang set-up is: 2015 GT, not a PP from factory but does have black PP1 wheel front and back, 255/35/19 Front. 265/35/19 Back. It is lowered front and rear, has a track setup, and has front (25MM) and rear wheel spacers (not sure size). Not sure exactly how much it is lowered (suspension installed by previous owner). Btw, I just returned to owning/driving a performance vehicle so I'm re-learning tires. I daily drive this in Phoenix. Rain performance is not a priority. The guy at Firestone mentioned getting matching front and rear sizes because I could rotate, but I'm not sure if they would rub or if that is a smart option. A YouTube video of a respected performance car reviewer said that matching larger tires size upfront on a Mustang would cause it to tramline on highways. But I watched a Steeda YouTube video of an owner of a GT350 and he deliberately put matching 20" wheels and tires on his 350. Personally, I like the look of wider tires and it would be nice to rotate front to back but not a requirement. All this said, I'm looking for feedback and suggestions for tire size. There are 275/35 and 275/40 19's available.