The purpose of the damper is just that, to reduce, or dampen down any vibration the turbo will naturally produce due to the high speeds it's spinning at, adding the fact that the way it is mounted effectively works as a lever on the manifold, and that it is roasting hot is a recipe for cracking to occur.
The damper was a bit added by Southampton University for NVH, noise vibration harshness issues, ie to make it nicer to drive, quieter with less vibration.
The turbo does indeed as said look more like a T4, and the lag you mention suggest this also, it is a really odd combination of T4 and dark green injectors, swapping to a T34 for example will see if boosting from around 1600rpm and full boost around the 2500 mark, in other words much more drivable in everyday use, with better economy.
I would when you get it take the ecu cover off, and check what chip it has, some idiots simply swap bits around with no though to it all having to work as a matched set of components, if say it is a T3 or T34 chip for dark greens the likely result of running a T4 would be a bucket full of bits that used to be your engine.
Make sure also it has had a fuel pump and fan rewire, these are essential on a cossie, the fan was very sub standard to start with when new and not up to the job.
tabetha