I can't say exact what you will also see at the cdj 850 because I only have some 2000 Nexus models. At my Players I can search see some entries of the tracklist of Cross and can search up and down at the list and load it directly like I would use it as normal with USB device or CDs and so on. I also see the BPM and the waveform. I can use all Buttons and so on at the CDJ and they are all working at Cross.
So I can say to the HID there is much functions but to you question to Serato HID I can't give you Information because I never used Serato in HID. To Cross I can say it is working very fast and it is working for me very well.
So I think the CDJ 850 will also run in that way like the 2000 models with some Little Display aspects. The Speed between Cross and the CDJs is very good and fast and also stable.
regards Christian
February 2012 Mixvibes DJ of the Month DVS 7, Cross (DJ), U-Mix Remote App (iPad & iPhone App)