Originally Posted by JCL

The BMW towing limits are due to the chassis, and not the engine power. The X5 doesn't have a frame, it is a unibody, so that is the limiting factor. There used to be a difference due to transmissions as well (5 speed auto had a lower towing limit on the 3.0) but that is gone now with the new transmissions.

I haven't towed with an E70. I did tow quite a bit with an E53 (3.0, 225 hp), and still tow with an E83 (3.0si, 260 hp). I also have a 535 with essentially the same engine as the 35i, but I haven't towed with it.

I don't think there is any concern over towing with the 35i. It will tow better than the naturally aspirated V8 used to, due to the increased torque at low rpm. 5000 lbs is not pushing it, I towed 4000-4500 lbs with the E53 3.0 at 225 hp, and it was fine. It depends on how many mountain passes you are pulling the trailer over, but for local boat launching I don't see a downside with 225 hp, and you would have 300 hp plus a much better torque curve.

Friends and family with Ford, GM and Dodge diesel pickups, particularly the most recent models, get used to that kind of acceleration when towing. Yes, it will be different with the X5, but towing shouldn't be a drag racing activity anyway. Just my $0.02.