Just test rode the Tenere followed by a new GS and GSA so got a better idea of how they stack up.
Hopping of my 2008GS the Yamaha seemed a shade lower (both seat heights are claimed to be 870mm in the higher position) but the bike feels narrower between the legs making it easier to manage at junctions and less of a chore to mount.
Once rolling the bike feels as light as the GS, it has a load more low down grunt, and probably a shade more mid-range, the GS "feels" faster up top, but that may be more down to peakier delivery, the only comparison I did was pulling out of a junction and gunning both to a reference point and the result was they both hit about 80 from a standstill in the same distance, with the Yamaha feeling far less stressed by the whole experience.
The Yamaha has noticeably higher gearing (4k = 70 on GS and 80 on ST) and vibrates far less, so whereas the GS is feeling quite buzzy at 85, the Yamaha is dead smooth at 100, despite this higher gearing my instincts and (rather unscientific) roll-on timing (one-one-thoudand-two-one-thousand) had them very similar in the crucial 50-80 top gear roll on region.
The Yamaha engine is definately technically superior to that of the GS, I have no idea how they would compare on a strip but I would be surprised if there was much in it, and my GS has de-cat headers and a accelerator module fitted which helps out the low-mid power noticeably. The other key benefit is the Yamaha is much better at fuelling so will cleanly pull from under 2k in top and I suspect will be easier around town and driving out of alpine hairpins, the GS lurches a bit below 3k and really does not like dropping below 2.5k.
Handling is very different though, my GS benefits from top of the range Wilburs suspension with adjustments for pre-load, rebound damping and Hi and Lo compression damping and have been honed to just how I like 'em. The Yamaha felt immensely slow to turn, so much so I stopped to jack up the pre-load a lot more which helped a fair bit.
The Yamaha was still nowhere near as chuckable as the GS it still felt long and low and reluctant to turn and braking into turns dived noticeably more (but for a fork bike was pretty good) it actually handled bumps pretty well, especially at the front (where the telever is a bit clunky) Unfortunately the roads were a mix of wet and damp with few completely dry corners so I never got to really get a feel for the Yamaha's handling.
What was most impressive on the Yamaha was the electronics, I have ridden BMW's with "traction control" and when it kcks in it feels like someone has turned your ignition off for a few seconds throwing you forward, before turning it back on again and sending you lurching backwards. On the Yamaha you do not feel a thing, the light comes on but apart from that you have no idea it is even operating. To test it further I stopped in front of a patch of mud on a back lane, 1st gear and full throttle could not faze the system it just drove through the mud as if I had slowly ridden through it manually - pretty amazing, similarly the ABS is pretty unobtrusive compared the to GS system.
Next up I tried the latest GS, this felt familiar, but the twin cam engine is noticeably smoother, it has noticeably more low down power and will pull cleaner from a few hundred RPM lower - it is still nowhere near the Yamaha for grunt or smoothness though, in the midrange it probably closer to the Yamaha than the '08 and up top probably beats the Yamaha, I think it beats the older motor at all RPM's but most noticeable is the extra bottom end and more free top end (plus it has an extra 500rpm to the redline) gearing on the new GS is identical and by 90 it too is feeling a little buzzy compared to the Yamaha.
Handling is pretty much the same as earlier bikes (I believe the sharpened the geometery in 2008) the demo had ESA and although quite good and very convenient I would say the Wilburs on my bike rides bumps better and provides superior handling, the best way to describe the difference is it has the handling of the later bike in sport mode, with the comfort of it in Normal mode, it just operates on a wider range. Also as I can tailor it more I have my bike a bit higher at the rear so mine steers noticeably quicker than the demo bike, with better feel and feedback.
I also rode the GS Adventure, this bike surprised by not feeling huge or unwieldy, it definately rode bumps better than the stock GS (as good as my Wilburs setup) but it was at the expense of feel and turn rate, it was not horrible, but lost that lovely sporty feel the stadard GS has, if I wanted to ride across deserts this would be the bike to do it on (assuming the FD would last the journey) but for my normal riding and touring to and around the Alps I would stick with a stock GS.
Not sure where that leaves me, the latest GS is not worth the £3k+ it would cost me to move from a (well sorted) 2008 model to a used 2010 model (earliest bike with new engine) if I was a new buyer I would prefer the later engine, but would not pay a huge premium to get it, the new motor is just a bit better in every way - overall a good improvement, but not revolutionary.
If being offered a "free bike" from those I rode today to keep and ride (but not sell on) I would take the stock GS every time, but if looking at the costs of @ £15,500 for a GSA / £14,000 for a GS / £11,000 for the Yamaha - (approximate costs if buying new today
if you put luggage on and have the computer / traction control on the BMW's to match the Yamaha spec)
As it stands I would not pay an extra £3500 for a new Tenere or an extra £6000 for a new GS which is where I am with the dealers, even a used Yamaha will cost about £3k more.
I may however do the swap deal, as I think the only way I could decide if I would prefer the Yamah is to own one for a while, and as this means niether of us lose out to the "stealer factor" it may be fun.