Sony 16mm F2.8 with VCL-ECF2 Fisheye Converter poor corners

I picked up a Sony Sony 16mm F2.8 with VCL-ECF2 Fisheye Converter to go with my a5100. I have seen some sources like Bluewater Photo's articles and videos recommend this setup. However the corners of every photos looks very out of focus for anything below F8. Even at F8 the corner performance is poor. At F11 the corner are better but still not at all sharp.

Has anyone else had this experience? Did I get a 16mm lens? I've heard the wide angle converter (VCL-ECU2) performs better than the fisheye but that doesn't seem to be saying much.

I've been using the VCL-ECF1 converter on my 16mm for quite a few years now, and as long as you remember to stop down, you shouldn't have too many complaints. The alternative would be the Samyang 8mm - you lose the autofocus, but with fisheye DoF, that may be acceptable - I haven't tried it yet.

I'm just trying this combo now - so far it's been in poor viz but I'm getting reasonable results at about f8