I think the 'losing multiple levels' often quoted is a perception. The Minimum Stack Level is always going to be a Flight Level equivalent which gives at least 1000' separation from the Transition Altitude. In Heathrow's case, it's always going to be a minimum of 7000'. On some low pressure days, because the UK uses whole Flight Levels, then there will be a need to round up the Flight Level to the next one which gives the minimum separation. That can lose you a bit of airspace (up to but not exceeding 1000') and technically (or numerically) one level. But it will never be more than this. Some people think that when the Stack Level pushes up to FL90, then you are losing 2 Stack Levels. This is not the case since FL90 is no where near an actual altitude of 9000' on those days.
Well, if for arguments sake the AMS FIR would have the same TA at 6000' and you have FL070 as the TL. In the AMS fir that would give me a holding capacity per stack of 18 aircraft (FL070 up to FL240). When the QNH drops and the TL goes up to lets say FL090 you lose FL070 and FL080. So that's a theoretical reduction in holding capacity of 8 in the AMS fir.
With normal traffic this wouldn't be a problem, normally there is no holding done and we stream everything from FIR-entry to the TMA-entry point. But when the Swiss cheeses align it could be detrimental. Had a situation last year where there was a Squall line passing AMS, and SPL-APP didn't accept any inbounds.
To cope with the inboundtraffic on the Eastsector we still got handed to us from Maastricht we had to Ad-hoc create not one but two non-standard holds because the normal ARTIP and NARSO were FULL up to FL240 (upper limit of our airspace). Have fun with that with even less holding capacity.
So therefor I would suggest to first go ahead and redesign all of Europe's Airspace, and then look at the most convenient Transition Altitude allocation. I really do understand the safety issues with having so many different TA's, but they are also there for a reason..