I think this is a simple way to think of whether you should make the switch or not.
The change in the midrange we can agree is very small (as it must be, its the same drivers, just a small reduction in distortion in the woofers which lowers the distortion that crosses into the midrange), at the level you could produce with cable change. For the ~$10k difference you will pay, you'd be better served with a cable upgrade, a component upgrade, vibration mitigation or power conditioner improvement. So I'd argue, if the only change between 802D3 and 800D3 was the midrange, it wouldn't make sense to upgrade from an 802D3.
So its all about the bass
The bass change is what's meaningfully different and where B&W put the extra money. Its not a change you could mimic with cables, components, or anything else. And I could believe for many audiophiles (music type dependent) it could be a game changer--after all, many people add a subwoofer to full range speakers to get that last bottom octave.
But there is a catch to the bass change--depending on your room you may realize the benefit of deeper and more powerful bass or you may actually suffer a *downgrade* in bass quality if it overpowers your room (as I found). I'd argue, its unlikely to be a wash, its most likely to be really better or really worse. If your room is big and you have freedom of placement, I am confident its going to be better. If you have a small room and/or inflexible placement options, its going to be worse. But, if you are somewhere in the middle, I think an audition in your room is critical.