Seeing that the RF2x alone is already £600, a combined, 3 position extender is very likely going to be at least twice the price of a single position extender.
A variable teleconverter sounds intriguing, BUT extremely expensive. In some ways that device could cannibalize sales of other tele lenses unless Canon limits its use to specific lenses. Either way, the combo will likely appeal to those with very deep pockets (the converter and the high-end tele lens) or professionals who need the versatility and can justify the expense. Either way I don't see it coming my way any time soon!