If any bike is vibrating at high downhill speeds, then there usually is something not aligned properly. I have a 64 cm Fuji Roubaix (aluminium) that would vibrate subtly, but only noticeably if I was at 23+mph on a slight downhill and "trying" to go freehand; some research revealed that 1. the frame design doesn't lend itself well to going freehand 2. headset had begun to wear (I had owned the bike brand new for a year before I was bold enough to try freehanding and noticing the issue).
I am sure that for any problem that may seem to be endemic to a frame model, you can always find a contrary example, as in this case. Before I go on, I'm in no way suggesting that you are not noticing .. "flex." I just want to be sure we are calling a spade a spade ..
Vibration/wobbling at speed is not flex. It IS a problem and it usually has a fixable source. Some [elastic] give under higher-than-normal force (pedaling hard out of the saddle or hard going up a hill, for example) IS flex. Too much of it and I suppose you could argue that it's robbing you of efficient power transfer to the drivetrain.
But let's think about it for a second, when going downhill, the bike will arguably see less force (than, say powering hard for a sprint or labored climbing); for all intensive purposes, even if you are pedaling, you are putting much less pressure on the frame with each stroke .. so it is unlikely that you will experience the same "flex" or bending of the frame. But if the bike is wobbling (kinda like when your car alignment is off, or when your tires need rotation or when your car tire air pressures are not equalized), then something is amiss.
So, just for clarity, is it "flex" or wobble?
You know, it could be too that the heavier you are, the less you will see this problem and maybe the fork IS that .. "flexy." Me, I'm 190 - 195 @ 6'3"; perhaps a lighter person will not be able to .. er, absorb the said wobble/flex/vibration. Does your fork have a suspension of some sort in it? I'm asking because maybe I need to start worrying about my fork as it is the stock fork on this TCR C2.
Good discussion, I think.