First, I didn't vote you down... Too much respect for you to do that. But just so I understand...
You're saying Catalyst drivers don't officially support 1600x900 at all, but will force 1600x900 when the card senses it as the native resolution via the monitor's EDID, correct? And man... Before buying these monitors, I knew it was a bit of an oddball resolution. I had no idea it wasn't officially supported. It makes me wonder if there might be any performance side-effects to using AMD/ATI cards with 1600x900 monitors as opposed to 1680x1050. Something like the card is still calculating based on the closest supported resolution, then filtering it down. Are you aware of any such problem?
Personally, I don't expect 16:10 monitors to support 16:9 resolutions, and vice versa. With LCD, any time you're not using the native res, things get at least slightly distorted anyway. The fact that you would also have to either stretch or squish to fit would just add more insult to injury.