Meade LX200 f3.3 convertors

Does anyone have extensive experience with these? I bought one last February
at Astrofest and thought that it would work about as well as the f6.3. What I
have found is that using it in the train:
LX200 - NGF-S focuser - f3.3 converter - filter unit with camera

produces severe vignetting at all orientations. I have rotated the camera
around to see how the vignetting moves and I can find one reasonably
symmetrical position - but the results are quite severe. With ADU readings of
~50,000 around the outer area the central region reaches 65,000 making every
image require deep processing. I checked the f.r in this position and it was
over f3, so not unreasonably configured.

Comments/advice welcomed.

Lawrence Harris