I always thought the Kelly was thicker sounding. I think the neck is probably closer to a Soloist neck, at least the SL2H. .
When I had the SL2H and the KE2, they all had JBs and Jazz combos. I hadn't really started dabbling in pickup swaps yet. So that may have contributed to the thickness of their sound.
The King V is the fastest I've played as far as Jackson goes. I love my King V. I've not played a USA RR1 only the KV2. I couldn't tell you how it compares.
Like with any guitar though, Jackson can be very hit or miss. I've owned four or five KV2s, a Korina, a natural flame and two trans black ones. The trans black one I have now is by far the best of the bunch. They all play the same that I can remember but the Korina sounded muddy, the first trans black V sounded like and ice pick on steel, the natural flame just sounded flubby. The KV2 I have now is perfect. From setup to sound, it's just great.
Test em before you buy em!