LS1 fuel pressure bleed off after pump replacement

[Tonner Matt]

is there a fuel pressure reg on the fuel rail?

There's no fuel pressure regulator on an LS1's fuel rail
It's a non-return system with an in-tank high pressure fuel pump, and an integrated fuel pressure regulator that is housed inside the fuel pump module assembly

In the fuel rail pic below, that circular item is a Fuel Pulse Damper......It is not a fuel pressure regulator

 

[VR38]

I have spoken to one mechanic whom tried selling me a damper claiming it would fix the issues.
Now idle stumble, and reverse stumble are creeping back in again.
today, I hit the track and at about 160 the car seemed to slow, I really expected to see 200 when I looked at the speedo.
I Am getting really frustrated with this,
Reading up lots of people have had issues with after market pumps and have either chased a genuine one in the box or replaced the pump in the module with walbro.