To be honest, I didn't see your last post -sorry.
I'm not an expert to graphics cards, and I can't imagine why you should have such bad perforamce. As I wrote before, I have the same system, but with an much older graphic card (GeForce Ti4200), and the Katana is fine on my system, as it has been designed for good performance (for such a high level of detail).
Make sure, you fly the variant you changed the canopy texture. Perhabs you changed it for the -80, and fly the -100.
Also you could try to rename the MM_DA20_VC_GL.bmp or move it out of the texture folder. Has the performance changed?