Didn't bother taking super in depth measurements, but took a friend's 235/40R18 RT660s off to install 245/40R18 RT660s on his 18x8 Enkei whatevers.
I have some 245/40R17 RT660s on 17x9 Apex wheels so lined some up for comparison purposes. Main takeaway is wider wheel = more effective contact patch all else being equal. My 245/40R17s are obviously worn from 4 days at VIR, but I suspect the corner profile would be noticeably more square vs new 245s pinched on an 8 in wheel.
245/40R18 on 18x8 vs
235/40R18 on 18x8
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235/40R18 on 18x8 vs
245/40R17 on 17x9
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245/40R18 on 18x8 vs
245/40R17 on 17x9
Stack of (4) 245/40R18 RT660 on 18x8 is 39mm shorter overall vs (4) 245/40R17 RT660 on 17x9 wheels.
No real scientific reason for posting, but figure it may be useful for fitment purposes to someone.
Realistically more tire is almost always better within reason, and same with wheel width. Run as much as you're limited to. I'd run a bit more tire than wheel for autocross depending on class, and more wheel vs tire for HPDE due to sidewall support giving improved tire (sidewall) wear. And of course these are generalities until you do specific testing.