Just like a n/a engine, the exhaust needs to flow, small turbos make them angry, but bigger turbos give more power. The idea of hi comp is so you have good, usable torque off boost and big power on boost, but remember it is proportional.
Tuning hi comp takes more time and effort, remember its all about volumetric efficiancy, and a decomp motor will always have a lower number, thus requiring more boost, and fuel for same outcome.
But why do people decompress a motor, its old technology, they didn't have such fancy programable ecu's in the 80's and 90's.. but they do now..!!!