Julian:

Could you please tell me the serial number of your K-80?

Another Canadian K-80 owner, also complained about a "compressor stall" sound at sometime ago. After further troubleshooting, it was this growling noise during acceleration from a well broken-in stock fuel pump (the pump became stronger then desirable at the existing setting), simply dial up the acceleration delay and down on pump limit took care of the problem.

But your situation does sound like you have a highly efficient K-80 over powering its own stock exhaust nozzle at max RPM at your desirable Canadian climate.
Remedy:
1) lower your max RPM, cuz you have already reached max thrust from the stock nozzle,
2) Reconfigure nozzle, enlarge it by 0.5 mm, (email or call me if you need one)

Meanwhile, we are preparing a thrust chart for K-80.

Regards,
Barry