I didn't want to bump the thread just saying thanks, but thanks for your post khwaja. Here is some more hopefully good info.
Kingston is sending what they call their "Fastest SDHC/SDXC UHS-I Speed Class 3 (U3) Card for 4K and Ultra-HD Video Capture" (link to page) for me to have a look at since I have this camera. I'll of course post file transfer speed numbers but more importantly for photography purposes, we'll get to see how much, if any, effect it has on the 70D's ability to shoot more RAW and RAW+JPEG without filling the buffer. As such, I needed to have some comparison numbers!
These were run with a 16GB Transcend Class 6 SDHC card. Class 6 is rated for a minimum 6MB/s transfer speed, making it just on the edge of acceptable for 1080p video, or so Wikipedia tells me. As this camera uses ALL-I format, that may or may not be a problem. So far it hasn't, but I haven't recorded much ALL-I and have the camera set to IPB to save hard drive space. Regardless of video stuff, this is a DSLR, not a video camera, despite its great abilities when shooting video. So what concerns us for the purposes of this thread is a) FPS and b) buffer performance. I'll save info about the new card for when it gets here, but here is the 70D's FPS and buffer performance with a far less than top of the line SDHC card.
You can see the shooting info in the graph, but for reference f/3.5 is wide open aperture on the EF-S 18-135mm IS at 18mm, so that aperture wouldn't slow it down. Obviously all of these numbers will be reduced somewhat in real-world use, but setting the camera like this and shooting with the lens cap on ensures repeatable performance so that you can obtain proper comparison numbers. I didn't note it in the graph, but the lens was set to manual focus.
As you can see, if you shoot in JPEG only mode, the camera does over its rated 7 FPS even with this kind of card. After 15 seconds of shooting, the camera took a total of 108 JPEG shots. As such, I consider the buffer to be not-fill-able when shooting JPEGs. The chances of anyone actually shooting 15 seconds at 7 FPS seem to be slim, so I didn't feel the need to go further and will consider that conclusive.
Once you get to RAW and especially RAW+JPEG, card speed will matter, potentially a lot. The interface can only transfer so fast of course, and after a certain point card speed will be greater than what the camera can transfer, but with a Class 6 card you're seeing just how limited things can get. At the end of the buffer-fill period, it had taken 18 RAW shots and 9 RAW+JPEG shots. After 15 seconds, those numbers were 25 and 17, respectively. Once the buffer fills, you're looking at RAW FPS of .57 FPS and RAW+JPEG of .44 FPS.
When shooting JPEG only, you can get a Class 6 card and be happy. When shooting RAW even, 18 shots before filling the buffer really isn't that bad. It will catch most of anything for any purpose for most people other than maybe dedicated sports or air show shooters. RAW+JPEG? Not so much.
I'm not sure when the Kingston card will arrive nor when I'll get it done, but these numbers should increase with that card. You'll just have to wait and see how much when the time comes.