A longer coil doesn't make it produce more heat. If it's using 300 watts of electricity it can create no more than 300 watts of heat. Resistive heaters are usually 100% efficient. If you have 2 heaters are are both drawing exactly 300 watts of electricity. One with a 6" heating coil and another with a 12" heating coil they will both still add 300 watts of heat to the water. There will only be one difference between the two which is the surface area to dissipate heat. The 6" heating coil will just be hotter than the 12" heating coil, but there is no difference in efficiency.
This may help with having a cooler heater to stop the risk of burning fish...But I've touched heaters that were on, even short 300 watt ones and the glass was cool to the touch. Water is very good at transferring heat.
Also a longer heater may end up with a lower internal temperature that limits the thermostat from falsely detecting that it has reached the desired temperature and turning off. In which case you don't really have a 300 watt heater, you have a 150 watt(Or whatever the duty cycle works out to) heater with a 300 watt peak.