I've now been measuring and taking into account the resetting of the SDR shift counter that happens whenever you switch between SPmode input and output - although I've not exhaustively tested those cases yet. Also there is some strange behaviour when you write to SDR while it is in input mode - it seems to skip one input bit in some cases.
As I am currently working on serial data transfer with the SDR (for OpenCBM and a 1570/1571/1581 drive): can you elaborate a little bit about what you found out? This seems interesting to me.
I am currently having the case the the transfer works for 3 to 5 MB, but every know and then, the transfer stops. To me, it seems that some byte was skipped. That's why what you wrote above sounds very interesting to me.