Hi Brian.
You got PM.
Stick with Bcredliner sound adivice, and only buy an OE or OEM MAF.
Buy the BOSCH item ref:0928 400 527,but try cleaning yours first!
If you disconnected the MAF and although car was better,the fact it still wouldnt rev past 3500 rpm,means its also another problem not picked up by your mechanic.
No harm in cleaning the MAF as Becreliner suggested.
First you remove the black engine cover behind the air ducts,then remove the MAF (need special torx key to remove the 2 screws), by undoing these scews and pulling it out!You then buy some brake cleaner and spray it on the metal elements and inside the MAF ,then immediately from some 30 cms (IMPORTANT) dry it with compressed air.

Your symptons ,could be EGR ,vacuum pipe leak or turbo itself with the exhaust vanes getting stuck.
Try disconnecting the egr Vac pipe that connects to EGR valve ,first and test!
No change!Then check vacuum system for leaks!First thing you do is remove in and out hoses on vacuum reservoir (this usually cracks) and join them together with a plastic union.Test and if no change,get your mechanic to use vacuum gun on turbo pressure converter IN and OUT hoses and on hose going to wastegate valve.
Let me know how you get on and we go from there.

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2003 BMW X5 E53 3.0D-Auto, Sportpack,full extras.
Oxford green/Leder Dakota interior.
56, 875miles @24th October 2024
Heavily OE modded,including full facelift grill install.4.6is body kit,wheels,exhaust tips and ext.fender flares
Non OE mods.Depo "custom" Angel eye headlights.