The 29" ish tall 235's on my rail buggy aren't doing my top speed or mpg and favors nor are they conducive to my overall goal of being low as fuck. So I'd like to go to a much lower profile 15 inch tire but I'd like to keep the availability fairly common and ideally I'd want them to be roughly the same diameter as my 165/80/15's up front.

Specs for most 215 tires state a 7.5" wheel as max. My question is will that extra .5" make for a dorky stretch?

Get a 50 series tire.

To do this you are stepping into specialized racing tires OR you are at the point of diminishing returns with height with a wider tread.