I think it enabled a finer grind without choking which is great for lighter roasts. It also helps get more consistent pulls. (But the line pressure preinfusion mod I did prior was really effective for that).

I replaced the rotary pump in my GS/3 with a gear pump.

I'm glad I did because it allows me to experiment with pressure profiling, though the results from that are somewhat subtle. The gear pump also augments flow profiling by allowing me to turn off the motor and pre-infuse at line pressure (which is reduced using a high-quality pressure regulator), and also allows me to do a very slow ramp to full pressure after pre-infusion. This lets me grind extremely fine, which is often necessary for light roasts and coffee roasted for brewing. That's a major benefit.

But this isn't the way Slayer does flow profiling. Slayer puts a needle valve after the pump and runs the pump at full pressure, so there's no profiling by varying the pump speed/pressure. Instead, the needle valve controls both the pre-infusion flow and the speed of the ramp up to full pressure after pre-infusion. There's not as much control over the pressure profile as I have, but there's a lot more control over the pre-infusion flow rate. (Note: the needle valve can be taken out of the circuit for darker roasts.)

Slayer happens to use a gear pump, but I don't believe it's really necessary. They allow the user to set the pump speed, which in turn sets the max pressure, but it can't be varied during the shot. This is similar to adjusting the bypass valve on a rotary pump.

One thing I don't know about the Slayer design is what they do about protecting the pump from over-pressure when the needle valve is cranked down. My gear pump doesn't have a bypass valve for that. I didn't get that option because the manufacturer states that it is strictly a safety device is not to be used to set the pump pressure. As I recall, repeated triggering of the bypass valve will void the warranty. So, I have to rely on the GS/3's OPV to protect the pump. But that means I can only put a needle valve before the pump, not after it.

I'm wondering what Slayer has done about this. Are they using a gear pump model that has a bypass valve similar to a rotary pump, or is there an OPV between the pump and needle valve?

It's defintely an option on the 1 group Slayer to change out the gear for an external rotary (costs $$)