I would not be overly concerned about the wires connecting the motor to the ESC getting hot, unless you think they are dangerously "set your plane on fire" hot.
When you say you want to upgrade, do you mean just making the plane faster?
When you say "replacement fan" do you mean you replaced the fan with a new stock fan or did you change the fan, for example, increase the number of blades (which would put the motor under significant pressure and would also inform regarding a suggested replacement)?
I was given a hand me down, slightly trashed one of these and was really unimpressed with the power. So I pulled the fan and put it in an 80% scale diamond. On 3S she was not great. Then I ran it on 4S (with a 40amp esc) and she goes like the clappers and the magic smoke seems to be staying in. So if your 40amp esc can handle 4S and you haven't changed from the stock fan configuration, this might be the easiest way to go faster.
DamoRC
