I use FastMM4, normally in full debug mode during development. If I turn off this mode should I arrange my app to go back to the Delphi (XE2) MM or is there a performance benefit in leaving FastMM4 linked all the time?
Delphi's memory manager (since D2007, IIRC D2006) is FastMM4, so you won't get any improvement in speed. The full FastMM4 might be faster, because you get improvements and bug fixes by using it straight from the source.