A lot depends on how you shoot.
When I first got my 5D3, I had real issues with clearing the buffer. My old cards were just not fast enough to clear that much data in a timely fashion and the buffer isn't that big. Shooting raw + jpeg also reduces the burst capability dramatically.
I shoot a lot of bird / wildlife stuff, also sports etc., so often find myself needing to shoot several short bursts in fairly rapid succession. With my original cards,I was quickly hitting the wall and having to wait to clear the buffer enough to take the next short burst.
I bought a couple of the Transcend 400x cards, as they are relatively economical (yeah, I would like the Sandisk 1000x cards but can't justify the cost) and a significant improvement on my old cards. The result was an immediate jump of an extra couple of frames in a long burst, as the card has cleared a couple of shots before the end of the burst, and the recovery time is so much better. It now takes around 16 frames in a raw burst at full speed before it fills the buffer, but it can clear that in around 7 seconds, so even with a full buffer it is clearing images at around 2 a second. I have timed it with a stopwatch and if I hold my finger down on the shutter release, and keep it there, it will shoot 17 shots at the full 6fps and then carry on indefinitely at 2 fps as it is clearing the buffer at the same rate.
As I say, I often need frequent short bursts at full fps but that card does the job pretty well, because I only have to have a gap of a couple of seconds between each burst and the buffer is clear again for the next one. If they are needed quicker than that, I can still get quite a few bursts in just a second apart before the buffer fills, then a slight lull in the action and it clears it again completely. If I decide I need to do a long burst and use all 17 shots at 6fps, The buffer will be completely clear again in 7 seconds or have space for a short burst after just 2 seconds.
Now, for some people that card may not be enough, I can appreciate that. Kin2son is very quick to say that it isn't suitable for the 5D3, but what they actually mean is that it isn't suitable for them. How much rapid fire shooting do you do? If you are blazing away like a machine gun for long periods, or shooting long bursts a couple of seconds apart, then it may not do for you. However, it can cope very nicely with anything I throw at it.
You say you shoot raw and jpeg, so I ran the times with the camera set for that and found the buffer fills with just 7 frames (compared to 17), and clears in about 4 seconds (rather than 7) whilst full it will continue to shoot at about 1.6 fps (rather than 2 fps). I suspect that this is down to the time requirement for processing the jpegs though, as there is no way the extra file size of a few jpegs will fill the buffer after just 7 shots. So, I don't know if a faster card would help there anyway.
Hopefully these numbers will help you decide if that card is good enough for you or not, we can't really tell you what you need as only you know how much data you need to shift and in what time frame.
Oh, if you put an SD card in it, expect everything to slow down horribly. The 5D3 doesn't write to SD very fast, so getting a faster SD card won't help, and only clears the buffer at that rate, not the rate it can write to CF.
