I'm running the V-spec as well. On it's 4th gallon now and compression still good.

The 421B is also a good choice, but we've had 3 instances of the cooling head breaking the top fin completely.

RB of course would be a great engine, having just won the championship.

I'd hold out on Sirio as there is a newer version coming out.

The thing I like with the OS is parts are WAY cheaper than italian engines where I live.