If you don't go over 60kph then the WR is probably going to be fine. I thought touring involved higher speeds where a 250 works pretty hard.
I'm not sure an XT660 manual tensioner is available or needed, usually. The design is one of the better ones, and has been around in form or another for years in Yamaha big singles.
A manual tensioner simply involves a threaded block bolted to the cylinder in place of the original tensioner, with a long bolt held by a locknut pushing against the rear slider. When the chain rattles, the bolt is screwed in a bit until the engine is quiet again, and locked up. The manual tensioners are great imo, but you have to be really careful not to overtighten the camchain.