In fact there has been for some time an independent test result on Fastestlaps.com with 3.8 0-100kph for stock 718 S. So Techart's tune promises to slow you down a bit! Interesting marketing there...
Porsche 911 Carrera vs Porsche 911 Carrera S vs Porsche 718 Cayman S vs Porsche 911 Carrera GTS - FastestLaps.com*
The larger issue: it's just silly that any tuner quotes their 'improved'' horsepower figure without also sharing what their baseline dyno run established as true hp from-the-factory. I think Black718 and myself (as well as some others here possibly) understand that '350' is a purely arbitrary figure -- to use that as the assumed baseline in any claim of tuning improvements, as well as not providing separate numbers for 'tune-only' independent of bypass-pipe, does not reflect rigorous engineering.
I suspect that tuners are using it to their advantage *not* to reveal baseline results, as it would make their tunes look relatively modest in terms of delta from true stock hp....
Note to OP and others seeking big 718 power boost from tuners, GT models or facfelift.gen 982: make sure you first understand what's already there in real-world performance potential with the current 2.5/VTG, irrespective of factory figures.
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scratch out 350 and write-in 390ps for the 718, and the metric/kph results on this chart are perfectly linear and consistent relative to power/weight. All cars are current gen PDK, launch control, and B6/B4 turbocharged powerbands, similar weight and Cd/frontal area. The 0-60mph number for 718 is 'manufacturers data' so replace that, and the quarter-mile data below, with Black718's stock results. Then this whole chart is just amazingly *right* for where a 390hp 2.5/VTG slots into the linear power progression of current Porsche turbo engines. btw you can see the source for any result by right-clicking mouse over the data box.)