Hi Darviathar,
In my experience when playing campaign, the Seleucids do seem to be underpowered and Macedon overpowered, with Baktria being not up to snuff strength wise and with the steppe faction being weak, which I believe should be made stronger, as I believe that the historical info does point to the steppe faction in RTR: PE actually having power to it. (I don't play it anymore due to balancing issues and also forgotten the faction name).
Consult: Rene Grousset's " Empire of the Steppes"
I hope with Marcus Camillus releasing RTR: PE 1.9 that these issues will be ironed out in that version and his 4 TPY mod & also of course the XR mod as well turn out as beautiful additions to a great conversion mod.
I have faith in this XR mod team that they will not let us down, especially if we have a bug post as well.

I know I'll post!
hellas1