Rob,
I would track down specific XT660 Tenere owner's forums and ask this question there. I'm sure there are a few forums for your bike and owners with experience in upgrading suspension.
Yamaha use mid level KYB (Kayaba) shocks on your bike. Generally not bad at all and pretty reliable in terms of NOT blowing out a seal ... but may be under sprung and under damped for loaded up, long distance travel or off road travel.
Most overlanders on just about any type bike re-do or upgrade their suspension, at least going with a heavier spring to handle increased load and harsher conditions.
Typical KYB standard shocks lack good rebound damping control, so you could either have your stock KYB rebuilt and upgraded (yes, it can be) ... or make the investment in an Ohlins or Wilber's, both excellent products.
I'm sure the guys on local Tenere forums will have more answers and experience on this.
I have Ohlins on both my DR650 and 1050 Triumph Tiger. Very expensive but GREAT performance ON and OFF road in rugged conditions. No blown seals/leaks on my DR650 Ohlins shock in 50,000 miles ... much off road in Baja. As always, YMMV.
Good luck!
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