The growing penetration of renewables calls for power generation and mechanical drive gas-turbine (GT) capable of quickly adjusting production and operate at part load. Aero-derivative engine architectures leverage the large experience from aircraft propulsion, have small footprint, high performance, availability and maintainability. Aircraft engines adjust power with fuel rate and shaft speed that go hand in hand. Mechanical drive engines need to change the delivered power by keeping the shaft speed under control to guarantee the operation of the driven equipment (an LNG compressor or an electric generator). Hence, the power turbine exhaust may deliver velocity and angle profiles that put the discharge diffuser in severe off-design with flow separations, high kinetic losses, and cycle performance shortfall. This paper describes Baker Hughes a GE company experience in the CFD assisted design and similitude scale-down testing of aero-derivative hot-end drive exhaust diffusers in multiple operating points. The diffuser inlet conditions reproduce power turbine exit profiles by using swirl vanes and perforated plates, the design of which is heavily CFD assisted. Predictions match measurements in terms of pressure recovery, kinetic losses, and exhaust velocity profiles. Different data post-processing and averaging are considered to properly factor in the diffuser losses into the overall turbine performance.