In order to actually achieve the full 300-psi at the cutting edge, you have to be using a tool that has specifically the exact resistance to flow needed to create that pressure. The pump's only job is to provide coolant FLOW, and then your actual coolant pressure varies depending on how much resistance it sees before reaching the cutting tip. The size of the TSC ports, the number of ports, those are the things that determine it more than anything else.
Specifically with a lot of the TSC endmill holders I use, they get around 150-220 psi depending on the orifice size. The only time I see it go as high as 300 is when using smaller drills around 1/4" diameter. If you use a big honkin facemill or insert cutter that has 6 separate .1"dia ports, you get easily half that pressure (or less).
In your particular situation I don't know if there's anything that can be done to the Haas OEM pump to make it get higher, unless you were to add some kind of an intensifier into the mix. hmmm