Like with many things automotive, emissions control has a lot to do with the specifics between the pumps. Really there are two types of pumps in use on the 1.5/1.6 engines, the '109' with the idle bump up and the '107' that doesn't have that. The idle control is different between the 107 and 109 pumps but either setup will work. In my opinion the 109 pump is preferred since it is easier to get the idle stable plus the idle bump up with the cold start handle is a great feature.
Other than that the majority of the differences you'll find across pumps of the same model will be around timing advance. Different p/n pumps may have different advance springs and/or shims, while some have solenoids to alter when the timing advance is used. Minor differences in dynamic advance won't make dramatic differences in the day to day driving of the car, but they may have an impact on fuel economy and NOx emissions. I don't know of anywhere that actually tests for NOx with mechanical diesels though. Particulate emissions yes, NOx not so much as I think the particulate kills the NOx testing machines.
Basically what I am getting at is if the pump is from a VW application and it bolts to the engine properly it'll work just fine. You might be able to play with the base timing a bit and find the "happiest" place for that particular pump on that particular engine which may well equal different base timing at a particular engine speed with a different p/n pump. Really though they are all pretty darn close to one another - the engines themselves are basically identical so it is not really any surprise that a pump intended for an "early" 1.6 will work just fine on a "late" 1.6 or vice versa. Even the 1.5 is not much different. The AAZ does have different injector technology so there's some things to watch for there, but amongst the "single stage" injection engines I think pretty much any pump from the same tech level will be acceptable.
Excellent explanation thanks!