Disco 2 Fuel Pump Pressure TD5

Hi all
My Bearmach fuel pump fitted less than 2 years ago is now only pushing out 50 psi (3.5) bar instead of the rated 58psi (4 bar). I have for a while had some vibration and what seems like a misfire above 2100 revs but the engine seems to full full power through the higher revs. Not sure if the lumpy running is the effect of the lower fuel pressure. Does anyhow know how low the fuel pressure can drop before it effects normal running?

A good pump should deliver between 5-6 bar and the regulator to keep 4 bar steady in the rail, if the pump's output is only 3.5 this pressure can drop under load so that pump is fubar even though this should affect higher revs too not just a middle range

Thanks
It's very confusing that it might affect mid range power I know.but it is a Land Rover. Guess I'll have to spend more money on a new pump.