As the operating Pilot, you are also not required to ensure they can. You are just required to understand the limitations. If you want to maintain these margins, you will never be able to operate piston twins with more than a couple of POB in ISA+ conditions.
CAO 20.7.4 states the weight limitations for a light aircraft in charter or airwork operations. Weight control is a responsibility of the Pilot in Command.
4 TAKE-OFF WEIGHT LIMITATIONS
4.1 An aeroplane must not take off at a weight in excess of the least of the weights determined in accordance with subparagraphs (a) to (d):
(a) a weight at which the take-off distance required under subsection 6 for the pressure height, temperature, runway slope (if in excess of 1%) and wind component along the runway, is equal to or less than the take-off distance available in the direction of take-off. Approved declared conditions may be used instead of actual pressure height and temperature;
(b) a weight which will permit compliance with the take-off climb requirements specified in subsection 7 taking into account ambient temperature and pressure height. Approved declared temperature and pressure height may be used instead of ambient conditions;
(c) a weight which will permit compliance with the en-route climb requirements specified in subsection 8;
(d) a weight which, allowing for normal consumption of fuel and oil in flight and taking into account either the forecast temperature and pressure or approved declared conditions, will permit compliance with the landing distance limitations specified in subsection 10 related to the longest available landing length under conditions of zero wind.
These are not certification requirements they are day to day based except for the enroute climb requirement which is ISA based. That is they must be calculated for the intended operation.
The Seminole (non-turbo), Seneca I and Travelair are examples of aircraft which do not meet the 1% to 5000 requirement and MTOW must be reduced for IFR charter and airwork in these aircraft. They were made to just make the FAA certification requirement of maintaining 5000ft in ISA.
The rule only makes reference to manufacturers data for take-off and landing distance calculations, and to factor them if there is no evidence that they have been. It also intends that the particular aircraft is to be considered, not type, not a fresh from factory aircraft etc... If you know that your aircraft can not comply with these rules you are operating it illegally.
The manufacturer provides basic data, it is up to the pilot to use the data apply factors and determine if the weight at take-off meets the legal requirements. How you approach this problem with what factors for a 40 year old piston up North is another debate.