The MiB 1 units offer nothing really over a fully updated (fw and maps) rns510 and to be honest the Kenwood deal had nothing to do with the rns being poor. The Kenwood was woeful to use compared to the rns510. If you take the discover and nav pro as just a sat nav with radio etc it's exactly the same as the rns. The latest rns even had ssd hard drives. I don't think the newer units were any better but by all accounts the mibii units are.
i had a Kenwood - it was a good unit for the price and that was it's point, unless you raided e bay, bought a 510 yourself and fitted it, you had to spec a 510 from factory, which when you started to add bits, was expensive, the Kenwood wasn't fantastic but for it's price it had lots of features, it was a good compromise.
I'd never describe the Kenwood as woeful, I used one extensively and again, never had an issue, would have rather had a 510 but it was a cost issue.
No, i don't think the MIB1 units are 'better' than 510's - what they do offer is a more modern look and feel but it is no worse than the 510. I think the graphics and look is improved, before we get misty eyed, 510's weren't perfect and 315's which i also used, certainly weren't.
James