Actually, you're both right-just in different applications. In any turbocharged application, short of one that has a CBE that is undersized, the turbo will always be the greatest source of backpressure. This is why we don't really see any gains with an aftermarket system. In this scenario, we assume that the final exit is located in the same place to safely expel gases beyond the passenger compartment.
In situations where you either don't care or need that safety (drag strip racing, manual/electric cutoff, etc.) or have a turbo located close enough to dump outside of the passenger compartment, then a short dump pipe will work better. This is mainly due to having a shorter length of tubing to have to move the air through-just like blowing water through a 3'straw as opposed to a 15' straw. The gain you would see would be less in overall hp/tq than it would be in faster spool to those numbers.
Obviously, on our OEM turbo's, the benefit would be moot. On an oversized turbo designed specifically for a racing application, it would be mandatory.