If BMW didn't want the public to know about this "V12" in a GM production car, then BMW would have never allowed GM to put it in. See their is this thing called the EPA. If its going to drive on US roads, the EPA needs to know the data on the engine, and of course, the tree huggers is not going to allow the EPA to keep this data secret.
If your Impala truly did come from the factory with a V12, I'd like to see some under hood pics of this, and scans/pics of the factory build sheet and RPO sticker please.
1997 Pontiac Bonneville SEi-SCRAPYARD
2000 Chevrolet Blazer LT 4X4 4.3L
2002 Chevrolet *-10 LS Ext Cab 4X4 5.7 V8