turbo theory is a complicated and highly mathematical science.
Small turbos will spool up quicker and provide boost faster, but generally wont provide as much boost pressure and at higher revs they run out of juice or even suffer cavitation.
Big turbos can power a starship - eventually, once its got over the turbo lag of spooling up.
Variable vane turbos are a bit special. They angle the vanes in the compressor side to a high 'pitch' at low revs to give fast and high boost pressures immediately, and then taper the 'pitch' away as the revs rise to maintain a constant boost no matter what revs are used. Means a comparatively big turbo can give good instant boost at low revs and still maintain boost with no cavitation at high revs.
Setting these up is a nightmare to work out and finding the right one to match your engine outputs and capacity is an art form. Theres been a chap on LR4x4 forum who was experimenting on fitting one to his 300tdi. I think he has used a couple of dozen different springs and lever arms to try and get the variable part to work properly with the engine and then his engine blew up.
Not sure if he is trying again or not. Was a hugely complicated thread with lots of maths and flow dymanics etc.
If the turbos you mention are of the variable geometry type and do work (and pretty much bolt on) i would be extremely interested! Followed by a trip to the nearest bmw scrappy
Mines had a slight fuelling tweak and goes lovely once on boost, but if i could get the boost to come in a few hundred rpm lower it would be bloomin lovely.