All,
All performance data is achieved under the most favourable conditions the manufacturer can find, not just light aircraft, Boeing, Airbus same same.
If the aircraft has an Airplane Flight Manual or equivalent in the Type Certification Data Sheet ( or equivalent), that is certified data, compiled to the requirements of the time/era of certification.
Remember that most FAA FAR 23 twins, unless they are certified to the "Commuter amendment", don't have a certified engine out performance at all.
Hence the old saying " A single engine aeroplane, with twice the chance of engine failure".
Interestingly enough, in the rare case of a forced landing in a "conventional" light twin, the incidence of loss of control is a statistically significant issue, and something that obviously doesn't apply to a single, including C208/PC-12 and so on.
Many pilots, depending on experience, would be well advised to treat an engine failure in a light twin as a single, unless the god's and the conditions are really on your side.
Tootle pip!!