Helical gears are usually recommended when you're trying to increase FPS rather than RoF. They have a greater meshing area on the gears which allows them to consistently produce stringer torque. If you're after increased RoF I'd stick with good quality flat gears (SHS Super Shooter, Siegetek, Lonex etc). There'll be plenty of other things to absorb your cash if you're going down the high RoF route!