I wouldn't think so, but I don't know the power transformer specs. Here are some things to consider.
The heater current draw for a 6550 is 1.6amp, meaning at least 3.2amp additional heater current would need to be available for the additional two tubes from the heater windings - above and beyond what the current tube set is consuming.
Putting the new tubes in parallel with the existing output tubes will increase current handling capacity - for reliability since the load will be split between 3 tubes instead of 2 for each side. This might bump up the total current requirement from the high voltage B+ to the center tap of the output transformer, but not drastically. The main limiter of current here would be the impedance of the output transformer primary.
If you expect to get more power from the additional two tubes, then you would have to swap out the output transformer to a primary with a lower impedance... and you would probably have to increase the operating B+ value (power is current times voltage, and a lower impedance primary would drop less voltage across it).
So... for more power from the tube addition, I believe you would have to swap both power and output transformers to achieve it.
When all is said and done... the sad thing is that your ears won't notice much of a difference with only a 50% increase in power output.
