Our performance numbers come straight from aircraft manuals and have been verified by multiple pilot advisors. If you can provide data to support inaccuracies in specific conditions, we are happy to make adjustments. But a youtube video with unknown weight and thrust settings in unknown weather conditions is definitely not an authoritative source upon which we can base wholesale changes to the flight model.
Keep in mind that our takeoff calculator uses the Airbus numbers - Individual airline operators may have different performance calculations better suited to their specific needs, including more aggressive engine de-rates on takeoff than the baseline numbers.
While it's true that the A340 does not have the same rocket capabilities on takeoff of a 757, it does have certain performance advantages - for example, Air France enjoyed operating them to SXM because the engine-out climb performance meant it could make a non-stop return to Paris, whereas the twin-engined A330s either required severe load limitations or a tech stop in Curacao to make the crossing 🙂