If the car wasnt mapped, the VNT mapping is going to be completely hopeless, so isnt representative in any way of how the turbo should be performing. VNT mapping is CRUCIAL in getting the best lag-free performance from a non-standard turbo. I've seen hybrids performing hopelessly with huge lag because the tuner hasnt properly dialled in the VNT map.
I think the MRC car was on 0.260's, upgraded intercooler and 11mm pump head like you say. However the engine is internally stock. As i said before, its up there for being a perfect bolt-on upgrade for a daily driven TDI. You can go silly with forged internals etc, but it just snowballs into a rediculous amount of money, and once you've spent all that its a waste just to fit a BV43, when you should be installing a 2260 and going for BIG numbers.
Sticking to a smallish turbo and external only mods keeps the cost down, and WILL produce good results. Sure it might be marginally more laggy than the stock turbo, but you've got double the power and a massive spread of torque, so a bit of lag below 2k really doesnt matter.
Not sure why you think the engine will magically "surge" if you havent got the big nozzles. If its mapped properly it'll work just fine, it just wont make 210+ hp on the standard parts.
If it were my engine, i'd start by sorting out the turbo, intercooler and exhaust getting all the ground work in place. Then slot in the 11mm pump head, 0.260 nozzles and get it remapped by someone who actually knows what they're doing.