Just to return to the perceived difficulties some have with applying separation from terrain and obstacles which are shown on the chart as AMSL, when using the QFE.
Since we are talking about MATZ airspace, we must be talking about UK airfields. And since that is the case, I know of no UK military airfield (or otherwise) below sea level. Leuchars, Lossie, and Kinloss are probably the lowest you will get. Therefore if an obstacle or terrain is showing 1800' amsl on the chart and you decide you'd like to overfly it at not less than 2800' indicated on your altimeter, then the absolute minimum altitude you would actually overfly it at, if using QFE as your datum, would be 2800' PLUS the airfield elevation. In other words, the separation distance will always be more if you are using the QFE. So what's the safety issue there ??
Leaving the ATZ and entering another piece of airspace, or not as the case may be, would work on a similar principle if you were intending to overfly it. The only time you would need to calculate things would be if you intended to fly under airspace with a base above the surface. Subtracting the MATZ airfield elevation from your indicated height gives you that calculation in a nanosecond.
They're both non issues in my mind.