Book an IR (not IMC) Instructor and do more training.
However you do not need turbocharged or de-ice for airways flight. i fly my 172 in the airways all the time. Very few flights cancelled due to lack of performance. I also fly a full de-iced twin and have only used the boots once this year.
The JAA IR is hard work to achieve but if you do a lot of flying is worth it. I have logged 200hrs of airways flying since finishing my JAA IR in June mostly in my 172.