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Colour me uneducated. Do I need this or is an Ultra enough for general DSLR use?
If you're only shooting JPG photos and RAW in non burst mode, then Ultra will be enough. For cameras that need to record 4K videos, you'll need a U1 or higher, like this card.
So for RAW burst there's benefit in the Extreme Plus?
Yes, but refer to the specs and recommendations of the vendor and model, as it'll be specific to each.
There's factors such as how much the camera can store in its buffer before writing to the SD card, and the size of the RAW/JPG images. So for some models, the bottleneck will be the camera rather than the SD card. i.e. some cameras might only be capable of taking 3 burst shots per second regardless of the SD card.
How How many raw burst until you see the difference with this card vs a 20mb read one?
Like I said. Refer to the specs of your model. If your camera advertises that it can burst shoot 10 shots per second in RAW, and your memory card isn't able to keep up, then your memory card is going to be your bottleneck. The increased speed also helps to speed up the general operation of your camera, like being able to display thumbnails of all your pictures faster is playback mode. Or spend less time writing out from the memory buffer to your SD card, which will speed up the time it will take to make your camera useable again.
The thing about SD cards is that some don't even perform at their advertised write speeds. Sometimes the memory card controllers in cameras won't even write to their card's spec. Read this for example: http://alikgriffin.com/best-sony-rx100-iv-memory-cards
Just buy this card. It'll cover you for everything situations. It's like asking whether it's OK to buy an SSD over a HDD.
Great deal. I just bought Sandisk SD Extreme UHS-1 U1 for $38 on Ebay last week. I would've bought this deal otherwise. Great (mandatory) for 4K high bit rate recording.