The fuel pressure should drop (to the regulator setting and then hold) when the pump stops because of the way the syphon jet system is plumbed in. As long as you are confident that the pressure will stay up under a full load (throttle and rpm) and you are not getting fooled by the gauge connection (happens more than you would think) then you can rule out the fuel system. It was the first thing that needed to be looked at especially since all the trouble started with work being done. Are codes coming back? is the bog and miss consistent ? An all of a sudden vacuum leak bad enough to cause that much trouble and not be obvious is a tougher sell for me. Maybe the CCV or brake booster ?? Have you driven with the maf disconnected (before starting engine) ?

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)