I'm in the process of comparing my new RP8/A2 to my old RP6/Exact2 and prior to that P3-2000/Dyn10x-5. For me the RP6/E2 was better in every area compared to the P3/Dyna. Using the Aria this might be expected. Nonetheless, the jump was more revolutionary than evolutionary.
For the RP6 to 8 jump, I would say the jump is more evolutionary, with the exception of the top end and overall speed of replay. The 8/A2 has an overall sense of complete musical freedom that sounds more natural and real by comparison, and this is ultimately heightened during complex musical passages where complete note separation wasn't audible with the 6/E2. [Note: I've taken Rega's suggestion and use 100ohm resistance loading for the Apheta 2, even though Dorgay thought <70ohm resistance gave better results on the A1. I took forum feedback on the overall superiority of the 2 vs 1 and assumed Rega got the matching exactly where they want it with 100 ohms.] The 8/A2 top end I think is the ultimate differentiator on the speed of replay being more natural, but that shouldn't be taken as a detraction on the bass. I haven't listened to enough bass heavy music, but early indications are that the 8/A2's bass while better, isn't as drastic an improvement as the treble was. This might have to do with how good the 6/E2's bass already was.
This leads to the natural question of how the 8, w/ Apheta 2, compares to the 10 vis-a-vis. It's interesting that Rega push the Aria as a good solution for both TT / cartridge combos. I would have suspected them to have pushed the IOS as the preferred solution for the 10/A2 more than they have thus far. Regardless, it would be very nice to see a forum member thoroughly compare the two decks with the same cart. A change of superior phono amps (i.e. Aria vs. IOS) could also potentially be an interesting variable to control in the context of this review.