And removes limiter for 200 mph top speed.

The BMW M4 GTS is pretty wild to start with, but you can guarantee that some owners still won't be entirely happy with it in stock form.

Step forward, G-Power. It has already developed a number of tuning packages for lesser M4s ranging from 520 horsepower up to 600 hp, and has now added its B-Tronik 2 V3 upgrade to the GTS, which remaps the engine control unit for better fuelling, sparking and boost. 

Then you can add a pair of new turbos with bigger turbines and compressors which, according to G-Power, are more efficient, reducing exhaust gas temperatures and increasing boost pressure. The result is 615 hp and 560 pound-feet of torque. That's 115 hp more than the standard GTS and 118 lb-ft more than the GTS.

That translates to a 0-62 miles-per-hour time of 3.6 seconds, an improvement of 0.2 secs. Which is only marginal, but with that much more torque the real difference will be in the mid-range. 50-100 mph should pass in a flash, but G-Power hasn't quoted figures for the intermediate increases.

As you've already paid to improve the power and acceleration, it would by daft not to spend a bit more to have the standard 190 mph speed limiter removed. Without it, G-Power claims a top speed on the far side of 200 mph.

Then you should probably spend a bit more again on the catalyst-free exhaust. And on the lightweight, forged, 21-inch wheels. Spec everything and the cost comes to €16,995 ($18,769), on top of the cost of the base car. If you can find one, as all 700 examples of the GTS sold out long before it even went into production.  

G-Power says four M4 GTS owners have already had the upgrades applied to their car, pretty much before they had even taken delivery.

Gallery: G-Power BMW M4 GTS