Sidi shoes

Not sure why Sidi and a few other reviews recommend going up a size from normal shoes. I'm generally a 10 (45) and genius 5s in 45 fit perfectly.

Sidi's shoe sizes are based on the length of their last rather than the length of the foot intended to be placed in the shoe. Sidi lasts seem to measure longer than lasts used for my "street" shoes. My size 12UK, for example, is 48 in Sidi. If you are really a size 10UK then, I'd suggest 46 unless you are a 10US.. that would be nominaly a 45. Sidi shoes are also quite rigid and use stiff synthetic materials that don't stretch..
You used the predicate "perfectly"... Not sure what "perfectly" means since I don't know whose size 10UK shoes you "perfectly" fit and I don't really know what last-- UK shoe sizes are based on the size of the last used to make the shoe-- you get a perfect fit in a 10... and even if the shoe is really is size 10 (a lot of shoes sold in the UK are not made according to UK lasts but in Asia on foot length) but perhaps a bit smaller.. width too plays a big role since without width measurements a lot of people grab for smaller, resp. larger, to compensate for their narrow, resp. wider, feet...
As an aside.. most people wear ill-fitting shoes.. a visit to an Oxford or Regent Street shoe shop will quickly demonstrate how neither customers nor sales personal seem to be well schooled in "shoe fit" (even Jermyn Street is not quite like what it used to be)...