I have only used a 23Hr once and it was on a demo shoot. However I do have the 28HR and use it often.
As already posted, the physical CF is the same part number for the 23 and 28HR you go from 72mm to 95mm. With the 95mm on you can place a slim CL-PL screw in type and not get any vignetting, however if you add a normal width one you will vignette (at least on the 28mm so I assume the 23 would be worse).
There is a Lee 95mm adapter ring made by Schneider, however it's hard to find and the one I was shipped was a normal not wide angle even though the box stated wide angle. The difference is that the wide angle is just a bit more open and you might get the Lee Hood without vignetting due to the ring. I took dremel tool to mine, and opened it up. Only problem is you have taken the anodized coating off and need to paint it with something to help keep out glare.
You can also purchase the Lee 95mm to 105 adapter, with hold up to 2 filter slots, this easily fits over the 95mm size of the physical CF and allows use of the 4 x 4 Lee filters.
No doubt on center shots, the physical CF makes a difference in that you will get much cleaner LCC's and much less noise in the corners. The 28mm without the physical CF is still much better than say a SK35 without on and many times the files don't seem to be too bad as it's shot dependent.
I have also noticed that if you use the physical CF and shift even 5mm you get a strange white vignetting before you hit the hard black (Black due to Rodenstock design). This white also will pretty much ruin a shifted sky as I have never been able to correct for it. This white vignetting shows up at 4mm and by 7mm is terrible. The black vignetting seems to show up at around anything past 7mm.
So if I plan on shifting, I take the physical CF off and live with the noise. You can take a 82mm Lee ring and modify it to fit over the barrel of the 28mm or 23mm since they are the same. This allows the use of the Lee wide angle hood and 1 filter. I also have the 105mm CL-PL adapter ring for my Lee hood and that works very well as long as you have the Physical CF off.
So long post, net, yes I see a considerable difference with the Physical CF attached on the 28mm and use it 100% of the time on non shifted work. If I am planning to try to shift I take it off and use the Lee hood setup. I have also noticed a strange reaction from the CF and a CL-PL in certain long exposures say 1 sec to 10 seconds in low light, where I see a light band along the top right of the frame. At first I thought this was a light leak, but it's easily fixed by taking off the CF. I get this on my Schneiders also.
Paul Caldwell