If it has to be open, the longer the pipe, the better. If you have only a few miles to go and they're on flat country, I'd say do it, but with a light throttle. The biggest risk is to hot exhaust valves and seats which might burn if cold air travels back up the pipe to the valves - which it could, if the pipe was short enough. I might clamp a 24" extension on to what you have, to be safe.
Run with the choke partway closed to keep everything cooler. Don't run it that way any longer than you absolutely have to.
