It may be improper, but he pulls better times than he did with 225/40R18 R-comps on the stock wheels (about the only other size R-comp you can get that will physically fit on our stock wheels - I think Michelin makes the Pilot Sport Cup in 235/40, but it's generally accepted that that tire is not as good as Hoosier or Kumho R-comps)...
If you want to stay in the Stock SCCA classes, you can't change your wheel width, and you can't even come close to being competitive without R-compound tires. This is why he runs that size.