I believe you are referring to the variable height MF holders with optional anti newton ring glass for the flatbeds.
I have an Epson V750, and have the Doug's holder with glass.
In the first place, the height adjustment of the holder lets you focus your film plane properly, and this alone is probably worth 40% of the resolution. Apart from this, the film is much flatter, though it is being only pressed down from above, either by the t locks, or by the glass.
I've had some minor issues with the t-locks, as some are tighter than others, so you should buy them in excess when you order, to use those with the best fit. Most of the time the t locks are sufficient to hold the film flat. Otherwise you can use the glass, which is ok, but I have found at times, especially when using the highest resolution settings (6400) some artifacts in my scans. I think it has been more a problem of the scanner rather than the glass itself, because I have modified my Nikon MF film holder for the Coolscan 9000, and I actually use Doug's glass to keep my MF film flat there (this has saved me about 400 eur necessary to get the Nikon glass holder) with great success and no artifacts.
To wrap up. I think Epson V750 is a very good scanner, it makes a nice job scanning medium and large format film (for up to 4-6 times enlargement), but it cannot do miracles, and if you want a 13x19 print that does justice to Leica lenses, you have to forget the flatbeds and go to the dedicated film scanners ( I've heard great things about the KM 5400).
Taking your pictures with state of the art 35mm film cameras and lenses to scan them on a flatbed is simply irrespectful of your imaging chain, its like putting a bottom of your milk bottle in the place of your enlarger lens.
From the little I've learned about the final MTF of your photo, it is a result of multiplication of the MTF's of single components - the problem being that you cannot go over 1, so each multiplication subtracts quality,
If you use the sharpest film and lenses to stay as close as possible to MTF=1, and then you multiply this by 0.3, it just does not make sense, it works better to get a scanner with a high MTF and use CV or Canon lenses, the final result will be better.