DNA Stage 2 Filter kit (~£80) and MTC single can system (~£180), plus a Power Commander (more expensive, but I bought mine second hand for £120). You could probably get some of the benefit of the PCV with a £10 variable resistor (look for "Kev fuel mod" on XT660.com).

I was stunned how much of a difference this made to the bottom end - at least 1000rpm more usable range.

Ok, so you're not talking an "out of the box" bike any more, but it's not a massively expensive set of mods, and there are other benefits (cleanable air filter, rather than the stock disposable one, and a decent weight saving over the stock exhaust).

Potentially complicates your luggage arrangements if you want to use soft panniers, as the can runs quite a bit hotter than stock. Ways of getting around this though, and some benefits - I've made a new undertray for mine which turns the space occupied by the stock exhaust into storage space. I've also got some plans for utilising the space on the right hand side of the bike which is no longer occupied by silencer.

I've never had a "big"/multi-cylinder bike of this sort, but I struggle to see why you'd need one riding solo. The Tenere will go all day at "Western Europe main road" speeds, and I'd have thought that would be as fast as you're ever likely to go on a RTW trip.

I have had a DR-Z though. Don't think I ever rode it more than about 20 miles without standing up on the pegs. Even if you did something about the horrible seat, it's still a much 'harsher' ride than the bigger single.