There is an additional plat chest for MVP. I don't believe there is a bonus plat chest for spawning the raid (there is additional magna). If you can solo the first 5-10 turns to build and keep MVP, then you can double your chance of getting null weapons.
On open raids you'll have to compete for MVP so bursty teams are a definite plus.
Dejnov.
P.S. I've also heard that snatch only affects gold and silver chests not platinum chests. You can't actually up your chance of null weapons with D'art/Mammon.