From what I've been told your towing ability is rated by your braking power. The X3 has enough power to pull a V20 + gear all day long but when it comes time to stop, that's where you are going to have a problem.
I have an '03 GMC p-up that is rated for 6900 lbs. I set it up for electric trailer brakes. I have towed a F350 that was around 10K plus a 2K trailer and the truck had no problem towing it for about 10 miles. What I was worried about was stopping. Even with E-brakes it was still a little hairy.
Like dbetterred said, short runs only. As long as your launch in close and you don't have to "stop and go" too much your X3 would be fine.
Almost forgot. You can add an external tranny cooler to help with heat.
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'78 Cuddy with '82/'88 Johnny/Rude 150
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