That's 40°C rise at 18l/min. Your boiler definitely isn't capable of that - you're installer has just invented that figure I'm afraid. Quick bit of maths says a 50kW boiler would be required to achieve that kind of performance.
Having said that, your performance tests do suggest your boiler isn't performing to its original specification. Is the boiler a long way from the meter? Do you suffer from hard water? Is your heating system water dirty?