I also think anything narrower than 11" in the back would be a downgrade all things equal (meaning similar compounds, definitely not comparing to slicks to OEM tires as there are so many other factors that may play a role)
I recently have my tires mounted and balanced on my Apex SM 10 wheels 18x10" front and 18x11" rear with (295/30/18 F and 315/30/18 R) As you may see from the photos they don't look like balloony even though 18" tires have more sidewalls and they're 10mm wider tires on the same wheel width (compared to 285/305 on OEM wheels). Ideally I would love to see some 18x10.5" and 18x11.5" custom fitment for my SS 1LE to run 295/315 or 315 SQ but oh well...besides since I don't have the Multimatics, even after grinding the knuckles (with my previous 1LE) 18x10.5 +14mm wheels were slightly rubbing with -3.0 camber
This is how they look. I will be testing them on March 23rd:
