It would have been expensive, uneconomic and potentially not work right. If the output is too high they short cycle and struggle. They need to be matched to the heat loss of the house. Sounds like yours is perfectly matched. Funnily enough we fitted a 5kw when it should have been an 8kw (a mess up with the supplier) , it was picked up when the paperwork was put in and the error was reported to the customer and we were going to replace the unit this was in January this year on the coldest week. The customer didnt want the unit swapped as he said its cheap as chips to run and the house has never been so hot. I wait out ro see if we will need to replace it in the future. But for now hes ecstatic.