Another aspect to this question involves the relationship between horsepower and torque. At the end of the day, how fast a car feels on the street likely has more to do with torque distribution than peak horsepower.
As I understand it, engines actually produce torque, and horsepower is a measure of how torque is sustained across the rev band. In mathematical terms, horsepower = torque x RPM / 5252. This is why when you see a horsepower and torque chart, the horsepower line and the torque line should cross each other at 5252 rpm.
The reason this matters is that peak horsepower--such as the 400 in the Q60 versus the 365 in the Stinger--doesn't necessarily give the full picture, especially for day-to-day driving. A car with a small displacement naturally aspirated engine can produce large horsepower numbers by making peak torque high in the rev range. However, such a car may feel sluggish at slow revs when in the wrong gear. An example of this is the classic Honda VTEC engines in the Integra Type R and S2000. If one was cruising in 5th or 6th gear at 2,000 RPM and needed a quick burst of speed, simply flooring the gas wouldn't do much, because those cars make their peak torque way up high in the rev range--think 6,000 plus RPM.
A turbocharged car with comparable peak horsepower to the Type R is the modern GTI, with the GTI having a slight advantage in peak numbers but the Type R having the advantage in power-to-weight. However, the relatively massive low-end torque in the GTI's 2.0 turbo would make a very different experience when flooring it in top gear at 2,000 RPM.
How does this all relate to the Q60 Red Sport and the Stinger GT? In my opinion, the difference in peak horsepower for both is a non-factor on the street, and both certainly have enough well-distributed torque and proper gearing to make short work of any realistic traffic situation. If I were choosing between the two, it would come down to other factors besides that number.
