The volley fire is spectacularly underwhelming. A far better perk or class ability would be something like the ability to change how your weapon works. Like for example the ability to switch from semi-auto to full-auto (For those of you who don’t understand the difference, semi-auto means you click and you fire once, full-auto means for as long as you hold down left-click, you keep firing. Full-semi-auto isn’t real). The marksman is supposed to be a trained soldier. Let him get the most out of his firearms.
The Ogryn’s charge ability is fine though the grenade is fairly underwhelming. Having it explode like a cluster bomb by default so a singular, large radius explosive and then having the perk instead be a melta charge so it does lots of damage in a very small radius would suit better. An alternate ability though could be to escape pinning attacks from elites like the dog. Considering it’s size, beyond the initial knock down, an Ogryn should be able to regain his feet.
The Psyker’s ability should be variable through the use of differing staffs or rods. Rods for melee focused psyker and staff for ranged attacks.
The priest’s special ability is just a variant of the Ogryn’s charge ability which doesn’t really suit the class’s theme. He’s a zealot, he should be able to use his faith to repel enemy mobs explosively or buff his or the team’s melee weapons or provide a temporary defensive buff to his team’s toughness. Another potential ability could be the ability to reduce the corruption effect either gradually or by killing elite enemies.