First, I don't use a Subal and digital BUT I used to use a Seacam with an S2Pro and the 12~24. I used the WidePort (very similar to Subals SWB Port) and a 35mm extender. I tried it with diopters and without. I found this set-up to be fine for distant shots at 12mm without the diopter, fine for closer shots at f/11 or smaller without the diopter until I got to about 15~16mm focal length below which it produced soft corners. With the diopter things improved somewhat and it produced very good results from close-up from about 15~16mm to 24mm but below ~15mm needed f/11 or smaller to produce reasonable corners. I never got around to trying it with a bigger port but it should have been better (I prefer a smaller port in rough conditions).
I've said this before and no doubt will again. When the field of view exceeds about 90 degrees (20mm on 35mm format), it is difficult to achieve good corners close-up (in both theory and practice). Also, no dome will work perfectly throughout a wide-angle's zoom range either and most people seem to opt for optimisation at the widest angle setting.
For the best results you should use as big a dome as possible, and just looking through the posts here, this seems to be the case. I would try +2 and +3 diopters if you can do so to see if either give a significant improvement too. I'd also try not to shoot much below f/11 if working very close.