I was wondering whether anyone had any success with running a psyker of this kind using the Only War rules. I know it can be done due to the open nature of system, but I was wondering if it was cost prohibitive and/or worth the investment. I've played one in our DH game, and had a blast. Anyone have any insights?

My group has a houserule; you can switch out one of your Characteristic Aptitudes for another, and do the same with the Special Aptitudes.

So you could end up as a Psyker with Willpower, Intelligence, Strength , Weapon Skill, Offence , Psyker and be fine.

If you don't go for houseruling, then it should still be fine. Just a little pricy.