Mine has been doing that since I bought it 2 1/2 years ago. It wasn't that pronounced and didn't do it all the time but there was a "better when the gas tank was full" for sure. I just changed the fuel pump and the throttle response is now better than it ever has been even with low tank. I think you are going to have a hard time diagnosing it with a pressure guage though. I think the problem is that the worn pump doesn't pump enough volume to come up against the pressure relief valve for the syphon jets which results in a slightly lower rail pressure since the syphon jets are fed from the low side of the pressure regulator. It would be nice to find a good diagram of the fuel system to verify this though.

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