Ok..
So if the temp in your room were to reach 14dC and your tank is 27dC you would need a theoretical 316Watts (continuous) to keep that temperature in the tank.
That is assuming 10mm poly and 12mm ply under both tank and sump.
Of course that does not include any added heating from pump, lights, etc.
I chose 14dC because that is when your 300W heater will no longer be enough.
If you were to put 25mm poly on rear and sides of your tank and on all 4 sides of your sump, your power consumption at that temp difference would drop from 316W to 181W.