Makes sense to me! I wish more people would remember this, considering all the "Shirou is supah powerful and op!" fics out there.
In addition to those factors, there's also the literally thousands of years of combat experience EMIYA has from being a Counter Guardian. Hell, even Nameless Hero seemed to have this going for him too, despite not being a CG (instead it's likely due to lol Moon Cell.)
Speaking of that, there's also said Nameless Hero to consider of course, but with that, there's a lot of weird Moon Cell stuff involved, and it's unlikely Shirou could achieve that level of power naturally, let alone that of EMIYA (since, as MHX mentioned, EMIYA's buffed by Alaya, and also the combat experience thing).
So yup, EMIYA is Shirou at his absolute strongest, but not necessarily something Shioru himself could achieve naturally in his own lifetime. Shioru's got a lot of hidden potential, but he's not nearly as strong as a lot of people seem to imagine him being.