I started with a Maxell 32GB CF card 400x from Fry's -- worked fine in the Tarken board. Then, I got a SD-to-CF adapter card that matched the one on Tarkan's web page (given before). It has the words "SD XC 4th Generation Extreme CF Adaptor" on it. I got it off the Amazon marketplace, but now I see Tarkan sells those, too. The Tarkan page has pictures so you can match visually.
So far, I've tried a SanDisk 64GB micro SD with SD card adapter (worked fine) and now a PNY 128GB SDXC Class 10 High Speed Flash Memory Card (on Amazon) with the CF adapter. I took care to use the HP USB Disk Storage Format Tool to format it to FAT32 before installing it. The HP software wipes the card and makes one partition out of it. The cards often come with two partitions by default which I think messes up the iTunes formatting if not dealt with.
Order of physical install: SDXC -> SD-CF Adapter -> Tarkan board -> iPod 5.5G.
As for read/write speeds, the latest 128GB version is fastest but still pretty slow. The Maxell was very slow (took several hours to fill 32GB), and it struck me that the 64GB micro SD version was slow too -- but I'm not surprised as there were three interface boards in play. Loading with the 128GB card took about 2.5 hours for 75GB in iTunes, then I side loaded some high-rez tracks for Rockbox which was faster (about 1 hour for 40GB). I don't plan to load/reload very often if I can help it.
About battery: Sorry I probably can't give a good sense for that because I don't run continuously and I try to charge so that I don't run out. I use an external amp with LOD pretty much always.