Well I got my answer, from Serato below....
Shame Pioneer couldn't admit the problem!!
Thanks for getting in touch.
You're correct - there are differences between the accuracy of the tempo slider changes between the DDJ-SR and SR2. The DDJ-SR2 essentially sends 'coarser' MIDI messaging than the DDJ-SR which essentially limits how precise each shift of BPM value can be per a MIDI note. An improvement to this aspect of the hardware, would require a firmware update and action by Pioneer DJ - so raising this with Pioneer DJ is correct route to follow.
Hope this helps, please let me know if you have any further questions or concerns.