300 psi and still slow

My buddy and I bagged his Mustang. We used 4 rs62 slam bags (comparable to a firestone 2500 lb bag) We are running all 1/2" lines and 1/2" parkers and it is on nitrogen. He had a regulator that would only let 200psi max come out. It was incredibly slow. So I said get a regulator that lets more air pressure come out and he did. We had it up to 300psi and it was still not all that fast. I need to know some suggestions to speed it up some. I already know that if we move the valves closer to bags it will help. He is also going to get a 7 port tank instead of just a manifold. I figure that if each set of valves has it's own supply of air it would speed it up too. What do you guys think. I thought 300psi should almost hop. His car is a 95 5.0 Mustang, do you think it could be too heavy to be very fast?