I guess this whole discussion hinges on how much weight could be saved by redesigning the structure of the Corsair for the lower requirements of land based ops only. If it is only a few hundred pounds, then the performance gains might be negligible but if 500 or 600 pounds then it makes sense. Quite a lot of FGs were built with no wing fold but I know of no data which tells how much weight was saved and what if any internal mechanicals were deleted or how performance was affected.
Sources I have list the empty weight of an F4U-1D is 8823 lbs and fixed wing FG-1D as 8695 lbs or 128 lighter, which would be insignificant to performance. I am sure this is a simple mod, just taking out all fold hardware and bolting the wing together and possibly removing the hook mechanism. A dedicated effort to Army requirements could double the reduction (similar to XP-60E), a slight improvement in climb performance but still not significant improvement over the F4U-1D.
The F4U-1 would make an adequate escort fighter, with similar performance to German defensive fighters and it certainly had the range. The F4U-1D would significantly improve performance at low and medium levels and a bit a high altitudes (but, of course, would have to have retained the internal wing tanks). I think they both would be effective in this role, but losses would be higher (no major advantages at higher altitudes). I don't think the Army was very interested in long range fighters until summer of '43. They had the P-38, which was then problematic, P-47 with range limits (which I think could have been easily solved by installing an extended range tank ala P-51B), and could get the FG-1. Raids deep into Germany could have been escorted by FG-1s and P-38s and, by fall of 1943, water injected P-47D-10 were available and, with extended range tanks, combined, with FG-1s, could have saved a lot of lives.
I still think that, in order to be truly effective in the escort role, a high altitude supercharger would be required for the F4U.