If you have an ATX chassis that isn't the cheapest in China, with a bottom-mounted PSU that allows for the PSU to be mounted with the fan facing downwards (with or without a PSU shroud), I'm willing to bet that a SHIFT unit will fit. Firstly Corsair uses physically smaller PSU-side connectors than other units, which makes them shallower so they take up less space; secondly they use flexible silicon ribbon cabling, as opposed to standard bulky and rigid sleeving, so the cables are far easier to route. If your chassis is able to offer 5cm between the edge of the PSU and the right-hand case panel (the one behind the motherboard) then you should have no problem fitting a SHIFT unit.
Yeah, it needs two inches of side clearance for the cables - I mentioned that already - but that's a lot fewer cases than you probably think it is. OCUK probably have the largest selection of major-brand cases on sale in the UK and maybe 20-30% of the 211 mid-tower, standard ATX cases will possibly accommodate a SHIFT PSU. Within the MicroATX, mITX, Cube, SFF, and other categories (they have 722 cases listed in total) that overall comptatibility of sub-10% demographic of cases that fit a SHIFT PSU is a pretty negligible target audience, especially when almost all of those compatible cases are so big and easy to work on that SHIFT PSU layouts add very little to the build experience, but do add compatibility concerns for that PSU if the build is moved to another case in the future.
Of the 10 most recent TPU case reviews, SHIFT is 100% incompatible with four of them, because they don't use an open basement with side access, so there's other stuff or a solid partition next to the SHIFT side of the PSU. Of the six remaining "traditional" basement PSU partition cases reviewed, only four of them are wide enough to accommodate the 50mm cable radius behind the PSU, and these are all new "MAXI" ATX sized cases that sprung up this (RTX 4090) generation and have enough room for 140mm cooling fans above the IO shield because of the super-sized 4090FE and its need for an even wider case to get the 12V HPWR connector into the top edge of the card with it's own stupid bend-radius requirements (otherwise you get melting connectors!). That size/type of case is a very new addition to the case market in general, and the industry has already started to correct it by marketing "smaller" high-end GPUs as a positive feature, and the proliferation of official and third-party right-angle HPWR connectors to reduce the need for 2" of wasted case width.
Don't get me wrong, when SHIFT works, it's a nice-to-have feature and I don't object to it being on the market at all as an additional choice. I'm just being a realist by saying that it's unlikely the wider case market will change to accomodate SHIFT layouts so it's going to remain a niche feature for a few specific scenarios. If anything, the latest slew of cases like the Hyte and it's copycats seem to be moving towards isolating the PSU entirely behind the motherboard in a portrait configuration (011D-style) which usually work worse with SHIFT PSUs than regular PSUs.
All the SHIFT models are showing as OOS for me on Corsair's website. As for pricing, I did pick up the 850W model for a big discount on Black Friday, but the RM series got bigger discounts (for example the SHIFT 850W cost me £120 while the RMe 850W dropped to £80 at one point IIRC).
You're comparing apples to oranges: The cheaper 850e is not the same class of PSU as the SHIFT, which is a rotated RM850X. The RMX series is a more expensive, premium PSU with better capacitors, cooling, PCB design, and better performance to go with the additional cost.
