I think you got this the other way around. Last i checked, AMD doesn't optimise for D3D9 anymore, and they were being criticised for it aplenty... but Nvidia still did do so. Would you like to point me to a source which confirms Nvidia doesn't optimise for it?
If you go over to GameGPU.ru and check performance of AMD vs. NV in the last 20 DX9 games, 7970Ghz crushes the 680. I've been tracking this for the last 2 years ever since Tom's Hardware made a ridiculous remark off 1 game that AMD cards have issues in DX9. In older games, less popular games, most recent OpenGL games and DX9 games, AMD's performance is better when looking at competing SKUs. NVidia tends to perform better when raw horsepower is in play (780Ti vs. 290X) or it's a very popular title, especially a GW/NV-sponsored title (COD, AC, Crysis 3). Of course none of this matters since you could have bought 2x 290 for the price of a single 780Ti and yet now 970 SLI is recommended over 980 but 2x290 was largely ignored by 780Ti owners.
I've been using both NV and AMD for a long time and I can't agree that NV is miles ahead. As I said, there are plenty of games where both have issues but NV users have a tendency to be quiet and sweep NV performance/bugs under the rug to maintain the allure of "superior drivers". If you read forum reviews and feedback from NV owners, there are plenty of reports of failed SLI scaling, crashes in games, poor performance, just the same as for AMD users. It just depends on the driver, game and GPU model.
For example if you played Arma3 for years, AMD was the only game in town. It took NV forever to get performance of that game up-to-speed on their cards.