This is probably the only game I know where your companions are better than what your character can achieve.
Not really, there is an insane amount of RT specific permanent buffs and some really strong items you get during the playthrough, + you can optimize your origin, world and class in a way that companions can't. You can easily outperform any companion at their job, if you build specifically for it from the beginning, with only exception being Cassia. Companions only look better than you on your first playthrough, because without meta knowledge it's nearly impossible to made optimal choices in character creation.
The origins, apart from Sanctioned Psyker, are not very strong. Also, bonuses from origin planets are not particularly important, and they certainly cannot compare to the additional psyker's school.
As for bonuses, practically all the bonuses I got in Act 2 stopped working in Act 3, except Champion of Abyss.
The remaining bonuses that you get in act 4 and which only affect the main character are small.
The exception is the halo device, but it has a few problems.
Firstly, from an RP point of view, using it is stupid and secondly, it doesn't work (surprise).
Most of the good items you can get can also be given to companions.
The only unique item I obtained that provides a greater bonus to the main character is the Eye of Joyeuse.
The remaining items could also be successfully used on companions (like Blooddrinker Staff) or do not work at all (psyker armor from Kabalica rep).