The VNT allows pretty much seamless, transitional between boost required versus need to dump, so its smoother than conventional - picture foot off throttle with conventional, wastegate opens, boost dumped, then foot to floor gotta rebuilt the boost which is more lag, the VNT bridges the transitions. They work exactly the same of course, just how to manages boost/dumping out the exhaust. The stock turbos run at something like 32psi at full boost, thats a lot of boost considering a petrol turbo will often desire half that boost for same effect, versus the derv which you can up the boost without paranoia due to the steel construction versus alloy.
In the diesel, keeping in mind how different it is from a petrol in basics, a remap affects a lot more easily, up the fuel flow in a derv and you'll get more power - and more black smoke, so to keep that balanced you want more air in too, the remap can tweak the timings, fueling and actuation of the turbo so they are all in sync with the new settings. The stock turbo is pretty solid and you can unlock substancially more power for a lot less money by starting with the remap option - definately consider that before throwing money at the turbo, you'll get instantly more power and efficiency for a lot less money Thats why a lot of diesels smoke a lot more after a remap - increased fuel related to no balancing increase in air.
Theres a reason these torque monster machines smoke like a fooker, everythings turned up to bugger
Thinking back to what I personally would do if going for significantly more power, I'd be considering adding a supercharger before I thought about changing the turbo, and would use the supercharger to reduce the lag of the big assed mental turbo I'd be sticking on. But now you're entering the realm of stage 3 tuning, great power but need to expect a consequence in the form of reduction of reliability and endurance of components. Can turn into a serious money pit.