Giant TCR geometry

I've been looking for a new bike recently and have test ridden a few. I've had my share of lower back troubles (chronic, just grin and bear it) and therefore prefer a slightly more relaxed geometry. I really liked the set up of the Domane SL. I also got on well with the Emonda H2 fit. 

However, I had my eye on a TCR and thought 'why not', so I test rode one today. The geometry charts tell me the M/L frame has a stack of 562 and a reach of 398 with a ratio of  1.41 - pretty racey.  I went to the LBS and gave it a good 10 minute spin and it was suprisingly comfortable. Granted, it seemed to have 50mm of spacers under the steam which would relax the ride a bit, but then the Domane I tested had 40mm spacers (with a stack/reach ratio of 1.56). I also test rode a cube agree with a reach of 394mm and the reach on the TCR seemed much closer. 

As everyone says, you can't beat throwing your leg over and seeing what works for you and in this case, the TCR seems great. However, my worry is that on a 50 mile effort, the racier geometry will take its toll. Does anyone (perhaps with similar flexibility issues) have any expereince of the TCR over longer distances? Is anyone else finding it staggeringly relaxed compated to it's stack/reach ratio. 

Thanks in advance.