interesting comment peruffo, do you know if other / most manufacturers do the same? actually, are you sure about the one for your bike adopting different tubes for different sizes? I wouldn't expect that, at least one tube profile for each size. maybe there is a couple, one for smaller sizes, one for biggers.. whatever, would like to know for sure.
as for your comment regarding weight differences, I'd add that for very light carbon frames as SS evo or the R5, Ive seen 20-25 gr differences between sizes 56-54.. So for the OP (size 51 or smaller) the weight difference should be even less, defenetly not worth it - as one cm extra of stem (Ive measured for the 3T arx) adds about 6-8 gr. so if you need 2 more cm in your stem, and a few cm of extra seatpost, that is likely to balance out the weight savings on the smaller frame.
having said all that, I 100% agree that fitting is paramount, but in my case, I used to run a 58 caad and swapped it for a 56 and feel so much better, faster handling, etc, being 1,86m tall (most bike fits I did would put me in between sizes as well). so the only real piece of advice I have is the one we less want to hear: try both sizes, same wheels and everything else if possible