It would be a huge improvement for films! As long as you have a sub as the M&K's cut off at about 80Hz. Are you using or going to be using a sub?
The B&W's will sound good for films, but if you compared them to each other you'll find yourself hearing so much more going on, and so much more detail. Once you're used to them, you can instantly hear where AV packages made up of hi-fi speakers fall down. They're pretty good with music too, depending on which amplifier you use with them. Which one do you have?
The M&K's are designed for one purpose and one purpose only. Most other packages are designed for another. They call it 'lifestyle'.
As long as the room isn't too big you should be ok. The M&K's are a 4ohm load, which can present problems to sub £1k amps at high volumes. If your listening levels are average to slightly loud, you should be ok.
How an M&K speaker sounds partly depends on the sub. The better the sub, the bigger and sweeter they sound. It's amazing how good the little Xenon's sound when used with an MX350.
Many have praised the S85's on here. They are a little more 'forward' sounding than many speakers, but i don't think you'll find them much more forward than the 601's. Even if you do, it's a small trade off for the extra detail you'll be hearing.
philp's comment is sound advice of course, but I bought all my kit blind and wasn't disappointed (other than comparing M&K to traditional HiFi offerings at the Bristol show)
As helicon says the key with M&K, and any sub/sat combination, is the sub. M&K's provide immense resolving power and dynamics that still make my 12yr old jump out of her skin. Criticisms of sub sat combos tend to revolve around integration of the mid with the bass. With a good M&K array this can be utterly seemless and IMHO renders the whole "full range" speaker debate a total nonsense.