I asked this question a month or two ago.
Apparently mk4 AEG pistons will work ok. I guess they have the 20mm diam wristpin, too.
I'd be curious to know what cast AEG pistons cost and what oversizes are available.
Do they use the same ring set as ABA pistons?
I'd give Techtonics Tuning a call and ask them your question. They sell a forged 3A piston set that they also advertise as working for the mk4 AEG. So, it must work the other way around, I figure, AEG pistons will fit the 3A.
One forum member stated that using the AEG pistons would drop the compression ratio .5 points from the stock 3a's 10.5:1.
I'd like to see where someone has cc'd the block with the AEG pistons to verify that.