Plainsman, what drivers are you using ?
We both run the same cards and my experiences of DX10 are a bit different to yours.

I don't get any noticeable FPS improvement in DX10 over DX9.
Also I've never been able to get FSAA to work athrough nHancer, only through FSX which is 2x.
And the biggie for me is that VSync doesn't work. Does VSync work for you ? I use TrackIR so I cant live without VSync.

Also there's the flickering runways & missing follow me arrows.

So the only things I gain with DX10 are the nicer water that I don't have set high enough to see to keep the FPS up and the cockpit shadows which are usually so basic or jaggie that I'd rather not have them.

My drivers start with 266, so I'm assuming it's the latest? I'm running at 1920 x 1200 x 32 rez, 16;10. I,too, had some runway flickering with DX10. Then I bought and installed REX2 and downloaded and installed the free REX Overdrive. REX lets you select your airport runway textures, clouds, skies, enhances lighting, and water. You can alter some or all of those things. YOu can select environment "themes." There is only a slight hit to FPS. The bonus discovery is that with REX2 the runway flickering is gone. I didn't expect that. I'll check, but as far as I know, I don't use VSync. Do you have your target frame rate in-game set to "unlimited?" or did you pick a number? I have mine set at unlimited.

I read somewhere that at low to moderate settings, you won't see a framerate increase between DX9 and DX10. But at high to very high settings you will. I snipped the following from a commment at another site:

"As Phil Taylor has rightfully pointed out in the comments section of the video, with bloom enabled and water on 2.X High people should see similar or higher framerates than with lower settings." I followed this advice. I have bloom enabled, water pushed all the way to the right as far as it will go. The funny thing is, on my PC, FSX with DX10 preview checked runs faster at 1920 x 1200 than it did at 1386 x 960. Go figure. I took this advice, turned it up and it's true. At that resolution, 2x AA is all you need to eliminate the last of the jaggies. I'm not a techie, so I don't mind taking advice from others.

Here's some forum posts from another site talking about the framerate issue. I"m not the only one: http://www.nvnews.net/vbulletin/showthread.php?t=133237

HEre's a quote from someone on Avsim: "On my system, FSX is about 30% faster in DX10 than it is on DX9 keeping all other settings equal. I'm in the process of benchmarking this in a more sophisticated way than just comparing an FPS here and there. Speed (as recently demonstrated with the ATI Catalyst drivers) is largely a function of the driver more so than the raw hardware. True, hardware is limiting, but a bad driver can truly make rendering speed terrible."

But another poster on Avsim disagreed: "I think you need to qualify this statement a bit. Like maybe on "your system" or exactly what your settings are that are allowing a 30% performance increase.

Using my exact settings under DX10 Preview from DX9 on my system(with bloom off), I can have as much as a 50% performance decrease under DX10 depending on area (30FPS DX9 to 15FPS DX10) just not acceptable. The only bonus is that Light Bloom can be enabled under DX10 Preview and it seems to have little to no effect on FPS."

It seems some are getting no boost in framerates between DX9 and DX10, fewer are getting better frames in DX9, more are getting better frames in DX10. And the variation in the boost in frames if DX10 seems to range from 10% to as much as 100%. I'm getting the 100% increase, fortunately. And the runway flickering is gone (because of REX 2).

FSX has some many sliders for so many options, it's hard to say why I'm getting a 100% boost over DX9. Like I said, I'm not a techie. I haven't tweaked anything because I wouldn't know how. My sliders must be set (accidentally) at just the right positions.

It's a HDR mod addon originally made for GTA but one build works with FSX:

http://www.enb.fstube.org/node/2

Well, maybe NVIDIA renders the DX10 PREVIEW mode better, that would make sens as it was halfway developped and maybe ACE were developing on the NVIDIA platform, who knows... it wouldn't be the first mention of NVIDIA handling FSX better anyways

BUT no, you can't crank up FSAA/FSAF as high as you want in DX10 and that's also a fact. DX10 forces the in-game FSAA/FSAF and will override ANYTHING you set out of FSX... quoting Phil Taylor on this:

Btw guys, how can i change the initial FOV and make the virtual cockpit a default view? I always hate to be in an external view or anywhere lese but not the VC. And the FOV is always too small. The cockpit and view angly is always off.

Dont like Microsoft at all.

They took the apha fade out of FSX now we get loads of s****y pop ups no matter how good our gear is, then they stopped supporting FSX.

The miserable sods did the same sort of thing to the Links golf community by removing fog and career mode after initially having them in the game, then they also stopped supporting that as well.

They can whistle for any money from me.