WEBF - the compound Merlin (which is the advanced Merlin talked about for MASC) would have primarily an increase in altitude capability and possibly endurance, but not markedly in speed. Still stuck with that stupid gearbox however (at its maximum rating at start of life!!) which puts the mockers on anything else.
I have yet to see how anything (short of running on SK bag or cross-decking the radar / mission system to a bog standard Merlin) can compete cost / capability wise with the most widely used AEW&C aircraft in the world (Hawkeye) which will be still in an existing production run when we buy MASC. All we need is to pick the right variant of JCA (Dave - C) - which would also buy us all manner of interoperability and future proofing.