Im not a scientist but i dont think you made a very accurate statement...
Yes fulltilts are a shorter boot in length, I just moved from an spk 27.5 to a fulltilt 28.5, with very very small difference in length (1 mm). However, I have way more room in my fulltilts and they dont crush my toes like the spks do, there is obviously a size difference for the foot area and i dont think the boot length has a whole lot to do with it.
As for if the fulltilts will fit OP, try some 27.5 on at a store first.. you'd hate to make a trade to find they dont fit your foot
my local shop i went in and asked for saloman kreations
next thing i knew i was walking out of the store with FT classics....good salesman