I don't have the same camera. My Panasonic LX100 says it records at 100Mbps. The first few days I had the camera I didn't have a U3 card and used a Class 10 card I had. The camera worked fine. Now that I have the faster U3 card, I see no change in performance.
My understanding is that cameras first record to memory, or a "buffer", and then transfer to the SD card. My theory is that since I was shooting short clips, the camera had plenty of time to move the clips to the card. I didn't test it, but if I had been trying to record 10 or 20 minute clips, it may be that the SD card would not keep up, the buffer would be filled and recording would stop.
Depending on what your shooting style is, you many not absolutely need the fast cards.
I don't think 64GB cards are necessary either. My book says a 32GB card will hold 40 minutes of 4K at 100Mbps. Unless you're shooting a show, play or lecture, 40 minutes is a lot of short clips. If I came back from a weekend of shooting with 40 minutes of footage for a 5 minute video, I would be overwhelmed. I am an amateur that does this for fun. Editing 40 minutes down to 5 would be a lot like work.
Last edited by bsprague; 01-16-2015 at 10:55 AM.