Not to hijack your thread, but I found a lot of the discussion very helpful in the search for our first tandem.
My wife and I are also soon to be proud owners of a shiny buttercup Burley Tamburello. We're a heavier team as well, about 360# combined at the moment (getting smaller each week though!). I was concerned about frame flex as well, given that I usually have to get the "beefer" model of everything due to my larger size (6'4", 210#).
This last weekend found us very motivated to test ride and talk tandems with the local tandem specialty store. (Insert shameless plug for the wonderful folks at Chain Reaction Bikes in Maryville, TN). The shop happened to have a Tamburello on the floor, and better yet it was on sale (last year's model). They also had a few other tandems in, but nothing that would really come close to fitting us. This Tamburello is a medium frame and we noted in the Burley cataloge that the medium sized frame is only about .8-.7 in difference in captain top tube and standover, compared to their large size. So after some mucking with the headset, seats, ect we got it in reasonably good fit for us. Off we went for our Tamburell test ride, and we didn't stop grinning the whole time! I've never seen my wife so excited!
I didn't notice any frame flex, the frame felt solid and easy to control. Granted, I don't have much to compare it to, but it felt safe and comfortable to us.
The frame isn't exactly the size we'd purchase if ordering one from the factory, but with the adjustments the shop made it is comfortable to ride. Our thinking is that we'll ride the Tamburello for a season or two. If the tandem bug is still going strong then we'll start looking at the major upgrade to a co-motion of some description. We're getting this Burley for a great deal, less than half of what we were initially willing to spend if we considered the Primera in our price range. Interesting to note, it was recommended due to our size that we ultimately look at Alum co-motions and not their steel frames if we find ourselves wanting to upgrade.
We should be picking her up this week! Now we start training on the tandem for our first MS 150 at the end of Aug!