I don't think VW would go down the remapping route for making a higher output 2.0TDI engine, they would almost certainly go bi-turbo, but how much would that cost and what would the effect on economy/CO2 be? If they could keep it above 60mpg and below 120g CO2/km for the manual versions and still chuck out 215-225PS I think they'd have a lot of buyers if it didn't add more than a grand onto GTDs list price. How much would that trample on GTIs sales? Quite a bit I would imagine, only the ardent diesel hater wouldn't consider it over a GTI still at 220PS.
There are a few tricks that VW could have used to make the GTI even more frugal such as cylinder deactivation tech seen on the 140TSI engine - probably would've added an easy 5mpg onto the fuel economy (but at what retail cost?). In the end they've placed them just far enough apart to minimalise cannibalisation of each others sales.
Being cautious, VW would probably reinforce a few bits of the engine like hardened pistons (that they used to do for R and Cupra 2.0TSI over the standard 200/210PS version - do they still do that?), so not all of the money is going on an extra turbo. Bigger brakes might also be something they add in, and then you might be talking another £1200 over the GTDs list price, would the extra sales be worth the effort or would 95% of the GTDers out there be happy with the standard model?
The 125d is a bit of an anomalie, is anyone else offering bi-turbo in this car size segment? I suppose it's a lot easier to justify the resultant list price on a BMW, even if it is a 1 series.
If VW upped the tech stakes for the GTD and GTI as my suggestions above, you'd be talking a 220PS GTI with 53-55mpg vs a GTD-R with 220PS and 60mpg. With 5mpg in it between them would many buy a GTD over a GTI?