I can say the Haldex system on 4 motion Caddy's has no issues whatever handling a whole lot more power - and there are plenty of tuning specialists who can help you in that direction if that is really what you want.
I suspect that the primary reason VW tends to limit choices of gearboxes and engine power on the Caddy models is to protect their profit. A quick look at the list prices on Parkers, suggests the scope for profit on models from their car range will be higher than their commercial vehicle range.
Inherently a person just looking for a 7 seater could be weighing up the relative merits of buying many different VW models - Sharan, Touran, Tiguan, Caddy, Caddy Maxi, T6, Crafter. By ensuring their van range offer only comes with lower power engines and limited creature comforts, it will help limit the diversion of potential sales to their less profitable models.