I used to cast aluminium condensers for Baxi/Potterton. I am not surprised that some engineers don't know about the new condensing boilers. It is a worry though when you are paying them a lot of money to service/repair them.
The condenser is a part of the flue really. Its job is to scavenge as much heat from the flue gas as possible. This is not a new idea - we had the same on some 1950's coal fired boilers at one place I worked, they were rather larger than domestic ones though
. If boiler return temps are high then the condenser itself will be hot and will take little heat out of the flu gasses. If return temps are low then so will be the condenser and it will be able to reclaim more heat from the flue gasses.
I can understand why it is important to correctly size condensing boilers. I think the important thing when selecting a combi is the hot water flow rate. Who want to wait an hour for the bath to fill up? Do condenser boiler modulate the gas flow to the burner relative to water return temps? If so I would have though having an oversized boiler would just run at a lower output but still give a good domestic hot water flow rate.