There are no other circuits to the barn now, no lights? You can't have more than one circuit from the house panel to the barn.

I would want to have a disconnect at the barn. This would also serve as the transition point from #8 to #12.
Are you sure you wouldn't want some lighting, too?
how about making it a 110/220V circuit to facilitate equipment for some future hobby? Putting in a breaker panel would give you flexibility in future modifications.
JMHO

Thanks for the quick responses...

I should have mentioned that there is another circuit to the barn, it comes from the same house panel and has a single 20A breaker at the barn and it supplies overhead lighting.

I have used that power on occasion (via one of those receptacle adapters that screws into the bulb socket), and as you might guess the voltage drops quite a bit due to the current draw. That's why I want to use the heavier gauge wire.

It sounds like to be code compliant I would need to disconnect that existing circuit? And if I did that, I would most likely want to go with a subpanel, it sounds like so I could run more circuits. The electrician who would install the panel (I would not try that myself) recommended that I de-rate the circuit to 30 amps on 8 AWG due to the long wire run.

Does that seem like the best plan to be code-compliant: remove existing circuit, add subpanel?

I will not comment on the de-rating, but yes, that is the way to go. A sub panel can cost as little as 20 or 30 bucks. Add a little more hardware for grounding and you will have a good 60 amps 120V or 30 240V, lots better than 20 amps 120.

You might be able to convert the 20-amp to a remotely-switched light. It could be handy.

Thanks for the advice, I will go the subpanel route.