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That is correct, they do look similar, however the fact is they are different cameras. Size (as in Megapixels) is not everything and actual hardware inside does matter and might perform differently.
As for the comment, as per my previous post, unless you have a very specific workflow, shooting burst continuously (which is rather unlikely underwater, and that is without even considering strobe battery life or recycle time), you really do not need the fastest cards. And for video, it is maximum 600MBps, which equals to 75MB/s, period. Math is math. Only exception being Sony mentioning "In slow-motion recording, the recording bit rate is higher than usual" but what "higher than usual means" is not clear. But even at 2x the max bitrate, we are still at 150 MB/s, easily into a good V60 territory.
Re the buffer, not sure I can fully understand this one long sentence, sorry. Cameras normally write pictures first to the buffer then to the card. If card is faster than the buffer, you can shot in one never ending spree, if the not, sooner or later buffer fills, then camera needs to stop taking pictures to catch up to buffer being emptied, i.e. pictures being written on the storage medium. For A7rV buffer size seems to be given on dpreview as "583 Compressed Raw files".
Either way, going back lets use the Transcend card as an example - even being on the lower end of V90 spectrum it can fit 49 uncompressed RAW shots at about 131MB each (based on the alphashooters data), before having to stop to clear buffer.
Will you get better performance (as in more shots before buffer fills and you have to stop) from a CF card for example, sure. But the real question is does your workflow require it? Are you really going to shot so many continuous bursts ? Rather unlikely, especially underwater. And if you do not, you will see virtually no difference. Photos/Videos will not be better because they are stored on a faster card.
Sony website does not mention "minimum" requirements". It just mentions cards that "can be used" or are "supported". It is in the interest if manufacturer to go with faster ones even if they are not strictly required. Just looking at the V60/V90 ones, despite having a given moniker, speeds of individual cards vary a lot, up to a 100MB/s on write between different cards, despite in theory being in the same class. Hence if you go and recommend lower spec storage class, you risk people getting a card that despite being in such class, its way on its lower end, and as such might impact camera performance in some scenarios. And then users will complain about the camera, which of course no manufacturer would like. Plus Sony is also a storage manufacturer, for both SD and the CF Express Type A cards.
Then you have the never ending upgrade cycles that every manufacturer so loves, you just must have the latest and greatest otherwise your hair will fall off and chickens will stop laying eggs - just look at consumer grade computer storage market nowadays "OMG you must totally have that PCIe 5 NVMe drive, it will change everything!" Meanwhile truth is, unless you run a database server for 300 users, you will see 0 benefit other than Windows loading 0.00001 seconds faster. PCIe 3 is already an overkill.
As for costs, I will skip percentages as that will vary wildly based on how/where gear was acquired, but costs of storage can vary a lot. Prices based on the same manufacturer where possible, except CF Type A:
-320 GB CF Type A Sony Tough - closest size to 256GB - 326 Euro - ~650 MB/s write
-256 Sabrent V90 - 224 Euro - 300-280 MB/s write
-256 Sabrent V60 - 54 Euro - 183-190 MB/s write
Now, one never just gets one card, as ideally at the very minimum one does want to have a backup one. So multiply each number by 2 or 3 and in some cases we have quite a sizeable chunk of cash. And no real gains. Money that would be perhaps better spent on a new lens, port, maybe better editing monitor, something that actually gives one a measurable improvement. Or at the very least higher capacity, as that would have a clear benefit of not having to juggle the cards around on extended trips, especially with such large RAW files. Or best, spent on actual diving, this is my favorite at least ;]
In other words, there is nothing wrong with spending but make sure you get the actual value out of your money .
Most of SD cards have some sort of waterproofing etc. though mostly to IPX7 standard only. Comparing CF vs SD durability, dunno, I have only stepped on an SD and it still works (Sandisk quotes impact proof as in begin run over by a 5 ton delivery truck, so a far cry from my weight to be honest). Yet to use write protect slider even once.
Card reader ("dock") is totally independent of the type. You can get laptops with SD, you can get ones with CF, or ones without any. That is more or less irrelevant, given the card reader sizes. There are even some attempts to create CF Cards (albeit type B) with integrated USB-C connector.
As for GH5 and V90 cards, let me quote somebody from reddit - "It's the equivalent of putting $2000 worth of high performance sport tires on a Kia Rio. The camera only needs cards that can meet or slightly exceed its write speed."
I can drive from A to B (assuming I am observing speed limit of say 130 km/h) in a Ferrari, then say everybody must have a Ferrari to drive reliably from A to B. But in reality, I can ride even a Multipla and achieve the same result, as long as it can make it to 130km/h. Of course it wont be that red and shiny as Ferrari, but it will get the job done ;]
Heck, if I would really ever have such burst demanding workflow, then scrap the cards, get the adapter, dispatch with the large heatsink leaving me with a tiny SSD instead, then thermal pad this to the inside of the housing - now a giant, watercooled heatsink. And I could then slap a 2-4TB drive inside and not worry about any capacity neither. Or go external recorder route, since clearly there would be a need justifying such cost.
As what Wolfgang thinks, I think only Wolfgang knows
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