i seriously doubt an 800 would make a huge difference in your system.
i'd also question whether a pair of 5000s would satisfy your "bass desires".
i use a pair of 5Ks for my rears. they don't suck by any stretch, but they ain't bass machines. i recently upgraded my 5000C to a 5500HC, and that was a wholly unsubtle exercise. i mean, serious bass upgrade.
if you want the bass for LFE/movies go get a decent sub. it'll be relatively cheap and it'll make your tootsie roll when "AH-NULD" kicks bad-guy butt.
if you want the bass for music, you should do some serious auditioning of the 5Ks before you assume they'll be all you need bass-wise.
i don't know jack about KEF, but a quick poke on the Net suggests you're comparing two very different speakers.
my two pennies.
good luck.
doody.