Boom, double post!
Fitted the SDXC cards and board last night. Was a bit of a nightmare to get the iPod open initially. I ended up breaking the spludger I was using, and worked the rest of apart with some guitar picks and a small flathead driver. Its well worth looking at the iFixIt guide on where the tabs on the front casing are, but its fairly easy to feel them once you've got something in.
With my iPod (5.5 30gb), I used one black foam pad (included with the adaptor) to make sure it didn't rattle around inside the casing.
The battery cable again, was a little bit of a nightmare, and the first time I put it back together, I didn't seat the cable correctly, so I got the Low Power error on the screen, but taking it out and re-seating the connector fixed that. I would highly recommend not clipping the iPod back together until you've ensured that its recognised by iTunes.
Once it was all working. I clipped it back together, and re formatted the iPod with iTunes, I was a little worried about the SD cards not being the correct format, but iTunes seemed to sort it out.
I went with a pair of 128gb Kingston SD cards.
Its now working great! Syncing lossless files (WAVs) took a really long time, so I left it going overnight.
So far, it seems good! Battery life seems the same, if not a little better. Its also much much quicker to turn on, and find files, no more waiting a few seconds to start playing!
Highly recommend the Tarkan adaptor, only took a few days to arrive, and superb quality!