FPS issue in Skyrim?

[Leopardson]

Hi,

I've been playing Skyrim for a while now on the Rift. I use the following mods to help keep the issues at a minimum:
Immersive FP - solves giant arm issues, allows extremely easy world scale configuration, makes archery and magic awesome
SkyUI + Immersive HUD - keeps only the HUD I want to see, lets me adjust it at will
Enhanced Night Sky - somehow, this solved the sky-being-too-close issue for me.
FNIS 4.0 - fixes animation issues with Immersive FP

The following are my computer's notable specs:
AMD FX-6200 CPU running at 3.8Ghz
ATi Radeon HD 7870 at default clock settings
8GB DDR3 ram of some sort
Reasonably fast 1TB HDD (7200rpm) in good shape
Running Windows 7

Here's the issue. When I switched from a Radeon 5770 to a 7870, every single game improved greatly in FPS, nearly doubling at everything including rift games - except Skyrim, which runs exactly the same as it did before.
I have shadows running at a low setting with a 2048x2048 map and a falloff at 1500 units with a LOD fade of 200. Deferred shadows are, of course, off.

My FPS in game drops to 10-20 in Solitude, Whiterun, Karthspire, and other graphics intensive places. When I was using the 5770, it dropped to the same framerate in the same spots. When I'm not running the rift, both the 5770 and the 7870 played Skyrim at a constant framerate of around 60.

Adding or removing mods or otherwise reinstalling the game makes no difference whatsoever on the performance. And, the real bugger - the performance is completely independent of graphics settings, unless I do something insane like set the shadow map to 8192x8192 and the falloff to 8000. I tried setting everything to very low and there was no difference.

I am afraid that this is normal behavior from Skyrim, but I also have a thread of hope that there's something in Skyrim limiting or choking the FPS. That is where you, oh wise MTBS3d users come in - Do you know of anything in the settings for Skyrim that would cause the framerate to be low independent of the video card I'm using? Or am I doomed with weak framerates?