the 7k was designed in the days of SD specification 1.0 so today's faster cards will not improve keyboard performance since this is limited internally by the hardware configuration. Likewise for audio they will be limited by the usb1 Panasonic card reader and show insignificant difference, however the built in SD slots of my various Toshiba laptops have worked for transferring audio, and at much faster rates than the Panasonic card reader with faster cards. For technics and midi data pc transfer faster cards will work provided you use a fast compatible usb2 card reader (there is a huge spread between speeds of usb2 card readers, slow ones ~3MB/s, the fastest ~19 MB/s).
Compatible 2GB cards can be successfully formatted and used for audio in Jukebox, but will show an incorrect % space in the 7k, but all the GB can be read and used in the keyboard. However a 2GB card will format as 1GB in the 7k and you will have to low level delete the partition to restore it to 2GB, a windows full format will just format the 1st 1GB partition again, leaving 1 GB unallocated and unusable.