Worth it to use fast TM on SW/DC dragonite to DT/DC?

I think it is worth it because it is safe to assume that SW will never be a good offensive move. At the same time they might buff DC or nerf Outrage at any time to make DC stronger.

If you have no better choice, I'd go for it. I have one SW/DC, 98 IV and will plan to use it first. Certainly doing that over my B/DP Gyarados

For out and out dps DT/OR is the best. But dodging is much easier with DB/DC. I would recommend you get DT though as in any case it's better to have a dragon type move.