A turbocharger (1) with an axial flow turbine stage (2) includes a compressor stage (4) using active casing treatment (50) for choke flow improvement. With switchable slots (52, 54), one of the slots (52) may be between the compressor's full and splitter blades (27, 23) and one of the slots (54) can be located downstream of the compressor's splitter blades (23) to maximize choke flow capacity. In turbochargers with a wastegate assembly, a pneumatic actuator (32) may control both a wastegate control valve (30) and an active casing treatment slot selection valve (56).