You much power its driveability..there is nothing wrong with 700kw in a rear wheel drive if you
1. Know how to drive
2.can make that much power and still have an engine that is easy to drive.
Often though it will end up a pig motor because big cams, you much lag etc making it uncomfortable to use on the street...but if you can make it with nice low down torque and an easy throttle the limits are endless..there's is plenty of 1000hp plus street cars out there that are RWD...actually I'd say the majority of them are, very few of them are what you would consider a daily though...But you don't need an extra diff, just big sticky rear tyres.