Hello Markus,
this could be a possible typo as the X488A-R5 drive translates into the Hitachi HUC109090CSS600 drive.
The drive itself has an interface of 6GB/s. Though, an official statement would be nice. 😉
I'm quite sure that the connection to an IOM6 won't be throttled, but I would not recommend mixing 3GB/s and 6GB/s disks in the same aggregate. (but maybe it's only a typo)
Regards
Michael