I've been playing with some shots I took today for the purpose and in some respects the results are better than Helicon. For me they make a whole bunch of marginal shots more likely to be used and they make it more likely that I'll keep and use the 28D but there are some provisos:
1) it's tempting to go too far with it so post-making these following examples I learned to view the file at 75% not 100% when choosing the sharpness falloff level. This is equivalent on my system to estimating a print nearly 60" wide.
2) for shots at F16 where you want the ground bottom left and right of a shot to be in focus from a normal height tripod, and you want infinity in focus too, you need less sharpening if you pull focus forward a tad on the lens. On mine about a third of the way back from infinity towards 1.5 metres does it.
3) the cost of the extra sharpening is, as you would expect, more noise/grain (and furthermore a grain differential across the frame which is subtly not quite right) so the technique is best applied to lower ISO shots. I have found that about the best balance of sharpness and grain for me is usually nearer 200% than the full 250.
4) further benefits (and slightly less need for the slider!) are to be had by applying it to a distortion-corrected image, because that applies a slight crop of the corners in any event, losing the weakest bits.
I have tried it on some old shots and it is far better at saving those that have had their focus pulled slightly forward rather than those focussed at infinity, which is where this lens tends to place focus in shots with subject centrally at any distance other than under its nose.
Here are three examples from today: they are from the bottom left of the frame. The first is a natural shot with focus pulled very slightly forward of infinity. The second is a Helicon from four shots at intervals of focus from infinity to just under 1.5 metres. The third is the first shot but treated with sharpness falloff to 250%. It looks messy here because I've had to compress the JPEG for upload and this has really exaggerated the grain. As a tiff onscreen it looks a little more grainy than the Helicon version but very acceptable.
View attachment 14461
View attachment 14463
View attachment 14462
Finally for reference the whole scene... which was shot at ISO50 MUP on a tripod with F16 at 1/50th.
View attachment 14464