vapilot2004,
I concur that the Airbus policy is to train thoroughly for the base model, then focus on differences training and indeed they have stated this at various conferences. Many operators have also strongly disagreed with Airbus at said conferences, especially those that only operate A321s.
I would disagree re cost as Airbus generate massive revenues (and profit) from Datapacks and the additional tests (whether flight or engineering) to support an A321 variant would be really quite insignificant and as pointed out by yannickue, they did provide A321 data in the past. (Many of the tests within the current datapack are years old and are as applicable now as when initially produced).
The issue of regular flight control updates is already addressed by bi annual datapack updates which currently Airbus provide FOC for 10 years although I understand this is being reduced to 5 years.
Don't quite understand the proprietary data concern.