I have never tried to use another focusing screen than the one they use as standard.
But for digital use it might be vise to find the best possible focusing screen?
What IS the best screen possible?
If you have one of the Acute Matte Hasselblad screens (and all the most recent 503s came with them) then you already have the best screen, IMO. There lots of variants on these screens, too, some with central split prisms etc, or plain. Manual focus with a MF camera on a high-res back is always going to be a challenge, that's why the H-Series cameras have very sophisticated AF systems incorporated into the design. You
can do it on a 500 or 200 series 'Blad, there is no doubt, but to really nail focus takes a great deal of care. It is so easy to get fooled by
apparent sharpness in the viewfinder, and with film we would get away with it (most times). With digital you can clearly see the tiny differences between focus at infinity and focused just a fraction closer in the file at 100%, which you simply can't see in the finder whatever screen you use.
Sometimes it is no problem at all. Shooting hand-held with the 250mm and a prism, wide-open at 1/500 sec, I can nail focus and sharpness pretty much every time. But with the 80mm close in at f8 or wider, I am often a foot or so out (usually back-focused), which is quite enough to ruin a lot of shots. The wider the lens, the worse it gets. You would think that DOF would be on your side as you go wider, and cover up your sins, but a 40 MP back has other ideas. I think the 80 MP back would simply multiply the problem x2.
Most of the time, if I am shooting hand-held, my 39 MP sensor is out-resolving the limits of my technique. Just now and then, I get a hand-held result which fully exploits all of those megapixels (see post below). If I use a heavy tripod, really locked-down, mirror-up and cable release,
then I start to see what the back is capable of. But there may well be a case for saying that, for general use on a 500 series camera, there is no real point in going beyond 40-50 MP. Interestingly, both Leica and Pentax seem to feel the same with their MF offerings, both of which are ergonomically designed to encourage hand-held work.
Just suppose that you could buy a 200 MP MF back for say $5,000 dollars (maybe you will be able to in 10 years time). If you just wanted to take what I would call normal photos on an old 'Blad it would be a nightmare, and quite honestly you would be better off with 40 MP.
John