I don't have a Ninja SLP air system, but I do have a Nitro Duck X-Stream air system with fully adjustable output. Now, output pressure is only one aspect of regulator performance, so running a different reg at 300psi may not truly reflect the performance of the Ninja SLP - keep that in mind.
Starting with around 4300psi, I was able shoot 1000 shots, with 1100psi left. However, below 1000psi or so, I was experiencing serious shoot down in rapid fire. I've not experienced that with tank output pressures above 450psi.
I would guess that there isn't a big enough difference between the marker's operating pressure and the 300psi output pressure of the tank, reducing air flow, and hence recharge rate.