Hi Edmond,
Did I understand you correctly that images which appear in-focus on your split image focusing screen, do not produce in-focus images on the film?
From what I understand of camera design, the distance to the fresnal (and split image prism) are the same as that to the film plane. If one is in focus and the other is not, either your film pressure plate, film flatness or the focusing screen are not in their proper place. All reputable camera companies build SLR (manual focus bodies) to work in this way.
The orientation of the screen is not relevant, because the screens are designed specifically for these SLR bodies and will have the correct distance to fresnal same as the distance to film plane. That is why there are specific screens for specific bodies and we cannot modify (eg. cut the corners, etc) of one screen and use it on another body. The thickness of the screen and which side the fresnal is on (top or bottom) have their design reasons.
An outside possibility is that your built-in diopter is not adjusted properly for your eyesight.
Floating systems (I assume you mean floating elements) do not vary the focusing accuracy of the body. Floating elements improve the color & contrast produced when a lens is focused near - but that's another discussion.
A bit on the AX and RX since I have owned both of them:
I have seen a few AX bodies where the AF focus does not tally with the split screen exactly. This is due to the alignment of the sensor and/or the focus screen being unequal, but its not a big deal and hardly noticeable in most photos. One of the reasons most consumer AF SLR's don't have split screens - confusing, unnecessary (among the many reasons....).
The RX & AX have 20% dimmer viewfinders than the Aria, 159mm, etc because the mirrors used in the AX & RX allow only 80% of the light to be reflected to the prism, the other 20% is directed to the AF & DFI sensors. So the issue is not the focusing screen, but the bodies. The same screen FW2 is used on the ST and RX2 (which has no DFI), I think you'll find it works at least as well on those bodies as your Can*n experience
Also, Can*n uses a totally different type of focusing screen, which is brighter, but gives less contrast - good for AF and low light subject framing.
On the 85/1.4, its a joy to use and not difficult to focus at all at 1.4. I've used it on a Yashica FX 103 also and had stunning results. If anything, the problems for accurate, in focus shots is the photographer's body movement. If the subject is still and you're using a tripod, the accuracy of focus of this lens is not an issue.
David's question about being in focus in the matt area but not the split image - i would trust the split image because that is what the designers want the user to use. The matt area best used only to frame the subject. There are specific screens like the FW5, used for close-up work and may be what you need.
Hope this helps and happy shooting.