Chromatropic - great to get some data and many thanks. Is it possible/likely that Canon will produce a firmware update to "uncripple" the SD card slot? I already have UHS-1 SD cards, my bottleneck seems to be the older (legacy) 300x CF cards that I use. Anyway, many thanks for all your effort and for putting it into the public domain.
This is likely a hardware thing. So it's not crippled, it's just the design at the time it was created. I'd be willing to bet that there's only 1 super-fast card 'port' on the DIGIC chip, and the other hangs off of essentially hangs off of the USB bus as a USB card reader, which basically limits it to 60 MB/sec theoretical max bandwidth, but because of the design age they only put in an SDHC (or was it SDXC?) which is the previous spec for SD cards which maxes out at ~30MB/sec theoretical performance.
As mackguyver put it, I'd bet that the camera pulls the RAW into memory, if it's supposed to write as RAW it generates the thumbnail JPG to write into the RAW file, and if it's supposed to write a JPG file it generates the JPG at the size & quality specified. At this point, it writes the file(s) out to the memory cards in parallel, and I'd bet via DMA (at least to the CF slot, which would help free up the CPU to an extent). At this point, if it finishes writing to the CF card, but the SD card is still waiting to finish it's write, it keeps that entire photo in memory (final output to both cards, so if JPG + JPG, the JPG for both, or if RAW + JPG, the RAW to the CF and JPG to the SD). So it essentially holds up a good chunk of your buffer while waiting to finish writing out to the SD slot. Once it's done writing that image, it clears out that particular images space in the buffer and you free up room for another photo in the buffer.
I think this is why, no matter what combination, continuous shooting for RAW+JPG is less than for just RAW. Both because of the extra CPU to generate the JPG, and the extra buffer space needed by the JPG.
And mikejkay, just go ahead and get a newer, higher speed CF card. You can get a quality Transcend 400x (90MB/sec read, 60 MB/sec write) UDMA7 compatible card for less than $40 from B&H. I've got them, use them quite a bit. Works great in my 5d3. It'll also just plain be faster than any SD card unless you regularly do a low-level format on the SD card as the UDMA7 supports TRIM, which lets the CF card clear out the blocks ahead of time instead of waiting until it actually needs to write to them.