Despite being limited to just 700 units worldwide at a cost of double the regular model, the M4 GTS quickly sold out after its announcement and now, tuning firms are beginning to present their upgrades for the track-oriented German coupe. Among the first to work on the car is G-Power, a leading tuner for all things BMW and rather than a simple ECU tune, the firm has implemented numerous mechanical modifications to lift power from 500 hp to 615 hp.

The turbochargers used in the 3.0-liter straight six engine aren't ordinarily capable of delivering so much power so G-Power customized the turbine wheel and housing. Additionally, a more free-flowing exhaust was needed to reduce backpressure so a set of stainless steel downpipes without catalysts have been fitted.

These upgrades have been joined with the obligatory ECU map in the form of the Bi-Tronik 2 V3 kit resulting in the 615 hp figure. Torque has also been dramatically increased from 600 Nm (442 lb-ft) to 760 Nm (560 lb-ft).