I used to use Avgas with an additive in my flat track dirt cars with great success. But a V8 has a long intake tract. It also runs at different heats and loads than dirtbikes. It also won't break the bank in a dirt car, as high dollar fuel sure will when your ride eats up about ten gallons for a heat race.
I've had alot of experience trying to make Avgas work in a dirtbike engine to no avail. It will run ok and is a consistant fuel, more consistant than pump gas...but you will not be able to achieve maximum throttle response and no matter what you do or how crisp you jet, it will still spooge like crazy. A man who doesn't understand jetting technique will drive himself crazy with Avgas and will always end up where he started.
Save yourself the trouble and don't even try it, unless you can get no other fuel except 87 octane, then I'd say go for it. 92 ron/mon pump fuel will give you a much better result in a dirtbike.