Haven't heard anything about a change of range, but I imagine that the bonus XP does decrease with the SiN buff. I know you still get XP for quests/bounties and legs while idling in town. Haven't heard about the XP out of range, but I can imagine you still get XP while out of range, just not as much as with the buff. Maybe they stealth changed it?
Yeah, I don't think it was posted anywhere. It might not even be intended.
With the game on T6, the lower level player still has that 1600% bonus from T6 and that applies on all kills while out of range (but in the same zone), they just don't get the extra 30% (~2080%) from the strength in numbers buff.
So, for that bounty power leveling method, players can split farm in the same zone to get through the bounties faster and probably level up the lower level player faster overall than if they had all stayed together.