@Josh | NFC I agree that the orientation in the S4 mini uses the "footprint" of the case (if it's laid down horizontally) more efficiently, and that there are definitely features you couldn't include in the mini with my proposed configuration, but there are absolutely use cases where this configuration would be preferable.
To quickly address the topic of risers, I found this riser cable for ~$15 more than the siliflex riser cost last I checked -
https://www.newegg.com/Product/Prod...cm_re=pcie_riser_cable-_-12-598-001-_-Product As far as latency goes, I don't have the link handy, but I recall LTT doing a video regarding the effects of PCIe risers on GPU performance where they found that the increase in latency is almost totally negligible for the applications S4 builders are likely to be interested in. I think they chained together a couple meters of PCIe riser cable without measurable framerate reductions. It was called "How many PCIe extensions is TOO many?" or something like that (I saw a result on a google search but 'something went wrong' when I opened the link). I don't want to totally discredit your arguments and experience regarding PCIe risers, but the proliferation of cases with vertical GPU mounts / PCIe bits and baubles for the mining crowd has made long risers surprisingly affordable; and fairly robust tests from a reputable enough source would suggest that the performance hit introduced by the riser would be far outweighed by significantly better thermal performance.
Now, space usage...
The biggest issues I can see with the reverse orientation would be the drive / fan brackets, internal power bricks, and tall GPUs.
Generally speaking, I doubt that very many of the S4 Mini compatible cards on the market are especially tall. As a direct counterpoint - there are a number of cards with PCIe power connectors towards the back end of the top edge of the PCB. I don't have access to my mini for a couple days right now, but I have one such card and the PCIe power cables are jammed right up against the chassis. I *strongly suspect* that these connectors stick further out the top than the PCIe connector sticks out the bottom, and that if the GPU were mounted backwards, these connectors would clear the mobo and fit much more nicely in the space between the mobo and the front bezel than pressed right up against the wall like they are now. I'd be curious to see the relative market shares of tall itx cards vs itx cards with power connectors in annoying places, but I don't think there's any chance we could know empirically whether support for taller gpus is a net advantage of the standard layout, or if top edge power connector space issues are a net disadvantage.
I think the brackets / fan brackets / power bricks / distro boards / cables are the really big problem with the reverse GPU orientation, and how problematic this is depends on whether a builder decided on the S4 mini because it's a practical, well designed, robust product, or whether a builder decided on the S4 mini because they just wanted a really cool computer. I'd like to think I had practical reasons for building in an s4 mini but there is frankly nothing practical or cost effective about my build. I'm sure that there are quite a few S4 builders out there who, like me, were a little awe struck when they saw one of your creations for the first time, and felt a strange compulsion to cram the very best hardware possible into a mini just to do it. Builders who are already packing high wattage flagship cards and processors into a 5L case that doesn't support conventional PSUs or appropriate cooling solutions for those devices aren't likely to be as concerned with support for 2.5" drives because NVMe is so much faster and sleeker and a lot of us have robust NAS solutions / super fast internet service / more spare hardware than we can ever hope to use. Brickless builds are neat and all, but I don't think there's any arguing the fact that G-Unique style solutions are the most viable power solution for high performance builds and incidentally require VERY little room inside the case. I'll try and nip this in the bud now because otherwise I'll be up all night raving about how much I like Gury's psu... Ultimately, users who shelled out for high end GPUs and CPUs probably don't need the brackets as much as more typical builders, are relatively more likely to have implemented a super space efficient 12V DC-DC solution, probably have power connectors in annoying places, and would probably be very happy to spend a good chunk of money on a part that will make their build much harder, and possibly make a small latency tradeoff for the possibility of improving thermals on an expensive component that likely isn't living up to its full potential.
This got a lot longer than I expected.
TL;DR: I don't think you should change the standard s4 design, and I don't even think you should sell variants of the s4 with alternate backplates. I'm principally curious what kinds of startup costs you'd be looking at to have your manufacturer (Lian Li, right?) manufacture the exact same backplate used on the standard S4 mini with the PCIe slots rotated 180 degrees. If the startup costs are low enough, I would think you could sell this modified backplate for a pretty fair sum to the S4 owners like me out there who view their builds more as works of art than machines. Hell, I would be thrilled to pay 3/4 what I paid for my S4 for a backplate that would gimp some of the expansion possibilities of my build, make it harder to change out components, and introduce all sorts of new stability concerns into my build if it improves my thermals. It's niche option, but I'd imagine the tooling / manufacturing process would be so similar to the existing backplate that you could quickly recoup the costs...
Hope I didn't offend / bore you too much. I adore my S4 the way it is now. These are just thoughts that've been kicking around in my mind for a while that I probably didn't organize or dictate very well :/