All of that is bunk. Hogwash.

While excessive CYLINDER pressure can cause gasket issues, any exhaust restriction is a disadvantage with a turbo. If the cylinder pressure is too great due to exhaust back pressure, exhaust flow is reduced, and less energy is absorbed by the turbo (which, in turn, can't drive the compressor). Excessive back pressure will absolutely SLOW turbo spooling. As the gas cools post-turbo, it contracts (gets smaller), which reduces the pressure to some degree. Larger exhaust, noise aside, reduces the velocity of the exiting gas, retaining heat in the pipe for a greater time. It's a trade-off. A turbo, to operate optimally, should have NO pipe. Just dump the gas right out of the turbine housing. But we don't do that, for some obvious reasons. Choose your pipe size(s) according to the gas you want to flow, generally based on horsepower. Anything in the 500 HP and less range won't benefit with larger than 4". Consider, all the late model HP behemoths are still coming from the factory with 3" and 3.5" pipes.