For road racing/autoX you would need some amount of slippage to get a smooth engagement, unless you are REALLY good at rev matching/heel-toe. In theory if you can rev match perfectly then there will be no momentum shift necessary to match the engine RPM with the clutch RPM. However if there is any difference in RPM than the momentum is basically taken from or added to your tires/wheels. Just think of the jerk when you engage the clutch suddenly while down shifting. Now imagine that jerk while you are corning at a hairpin turn and your tires are at their limit... I don't think the end result will be pretty. Basically a little amount of slippage will give you more room for error so you don't need to be that perfect at rev-matching. But like I said, if you are really good at it then more power to you and get the 6-puck by all means.
