The INTAKE leak is not likely to be causing power problems, as you've probably tuned around the leak, in effect taking it into account... though it can have adverse effects.

However when combined with a rich fuel mixture, it can make the engine hard to shut down as you are experiencing.

I have one SuperTigre that air leaks SOOO badly around the throttle drum itself that I can completely close off the throttle and the engine will just keep running until the fuel runs out...

The engine runs fine in the air, but I have to go running after the plane to pinch off the fuel, and even then it can take 40-50 seconds to shut off once I pinch off the fuel line... ugh!