If unplugging the MAF makes no difference to the symptoms then the MAF is not likely to be at fault. Faulty MAFs don't usually cause limp mode.
VNT turbos have a set of vanes in the entrance to the exhaust turbine. These vanes change the area of the inlet by tilting against the airflow, but are carefully shaped to avoid causing turbulence in the flow. See
this page, especially the amimated gif at the bottom.
If the car is mostly driven around town or in commuting traffic the exhaust and the turbo never reach full running temperature, and carbon deposits from unburned fuel (mostly caused by the EGR system) build up in the exhaust. These eventually start to obstruct the vanes.
The vanes change the area of the inlet to modulate the speed of the exhaust gas flow onto the turbine blades. At low revs the engine produces less exhaust gas, and the unmodified gas flow in the exhaust manifold is too slow to spin the turbine up. By reducing the area of the inlet, the gas speed is increased (same mass through a smaller area) and the turbine spins up to produce useful boost. The action of the vanes increases the back pressure in the exhaust manifold, but this is more than countered by the increase in boost pressure and thus engine power.
When the engine is switched off, the vanes are spring-biased to the fully open position, which is the position they adopt when the engine is at full revs. As the engine is started and vacuum is produced, the vanes are pulled to their fully closed position - which leaves a minimum area for the exhaust to go through. So the vanes go through a full sweep every time the engine is started, and don't stick at this point.
As engine load increases, the exhaust gas volume increases and the vanes are commanded to open up to keep the turbine spinning to produce maximum boost. Turbine speed and the boost pressure it produces are controlled solely by the vane position. In a clean turbo, the vanes reach their fully open position when the engine reaches maximum revs.
The vanes are being asked to move against exhaust pressure, which is loading them aerodynamically, and makes it harder for them to move. Carbon fouling can add enough resistance to vane movement for them to stick instead of opening properly. If they stick, as the revs increase and the gas volume increases, the gas speed through the vanes gets too high, spinning the turbo too fast and resulting in overboost, which triggers limp mode.
This is the most common cause of limp mode in the circumstances you describe.
It is sometimes possible to relieve the turbo by getting it up to a good high temperature for a few minutes - the "Italian tune-up". You need to be sure your car is in good condition, all servicing carried out properly and no cooling or lubrication issues. Find a piece of dual carriageway, preferably with an uphill gradient, and not too busy. Drive up it at 70mph in 3rd, which should have your engine close to maximum revs. Keep this up for five minutes or so - this will get the turbo hot enough to start burning off the carbon deposits.
This does work, I've done it in my old Toledo TDI 110 and in the present TDI 150, and in both cases it has eliminated the occurrence of limp mode at motorway overtaking speeds. Some people have spoken of seeing sparks out of the exhaust when doing this at night - this is the carbon particles going out of the exhaust, still burning.
After a few minutes of this, continue to drive for another 5-10 minutes at normal engine speeds, to allow the turbo to cool off gradually.
Chemical cleaners can work, but you have to get them into the exhaust turbine and out again without damaging anything. Each has their own way of going about it.
