I've not been a forum member for long and so just reading through some of the posts. Perhaps, due to the time since you last posted here, you have figured out many other tweaks with the Nvidia Inspector software.
Regarding setting frame rates for FSX, I've always been advised to set FPS limits using external software (such as Nvidia) & it has always worked for me.
My preferred method is to first set the frame rate within FSX settings to unlimited (bear in mind that by making any changes using the FSX settings it will automatically change your FSX CFG file back to it's default);
Next open Nvidia software. Before clicking on the 'spanner' icon, scroll down to the bottom area of the page & you'll see that you can set your FPS to your preferred rate. Select FPS rate & save. I initially set mine to 30fps but was able to increase to 40+fps. It's trial and error depending on individual rigs.
Click on the spanner icon and select the MS FSX game and proceed as usual. If you find 30fps ok, then repeat & try gradually increasing frame rates by 2 or 3 at a time until you find the best for your system. You can always reduce the FPS back if you need to.
Avoid making too many changes in one go. If I do make changes to the Nvidia antialaising or other settings, I first either take a screen shot of my current settings, or take a photo with my cell phone so that I can revert back if not happy with the new setting. And I only change one setting at a time.
As I say, it's works for me and so I hope that this information is useful (albeit late).