yes i have came across a cracked housing a few times in the past, they dont normally make a big drop in boost unless the crack is comming from the wastgate penny seat area, they normally crack in the exhaust gas inlet union around the turbine caseing and can be more noticable by being a very loud turbo (i can describe the sound to be like a hoover lol) Also another place to check is the seal between the compressor caseing to the backplate that bolts onto the bearing case, its only made of paper gasket type stuff and they are known to crack and leak, easy change by unbolting the compressor housing which can be done without removing the turbo from the car