Wattage on it's own does not equate to loudness. I've seen people make statements along these lines that create confusion on how this works such as "My receiver pushes 500 watts". The receiver is a source of power for the speakers, but the speaker takes the power. This is why if you take the receiver past it's operational limits the speaker will continue to take power even if it is not present and the receiver will overwork itself.
Now believe it or not the difference between 100 watts and 200 watts is a mere 3 decibels. When you look at the scale of decibels and how it relates to sound pressure in your space you will realize quickly how minor this could be, or how important it could be. For every 10 decibel increase it is perceived that there is twice as much sound by human hearing standards. A 6 decibel increase is twice the measured sound pressure. Finally a 3 decibel increase requires twice the amount of power.
Assuming your speakers are 90dB 1 watt / 1 meter, meaning your speakers achieve 90 decibels of sound pressure with 1 watt at a distance of 1 meter, then if you increase the wattage to 2 the decibel level should reach 93dB. In order to reach double the percieved level of output from 90dB, 100dB, you would need about 10 watts. This all assumes this is a constant single tone or series of tones with a distinct pattern are being used to achieve these output levels. In reality your speakers could demand 1 watt and suddenly demand 30 watts for an explosion then go back down to 1 or 2 watts again after the impact has faded. Most of the benchtests are constant basis or long term ratings. Peak levels are significant as well and most modern receivers can handle getting you sometimes over 100 watts to 2 or more channels for a few moments without having noticeable impact on the sound or operation of the receiver.
If you had speakers with a complex load or were very inefficient the use of a much more powerful receiver or even separate amplifier would indeed be quite useful. I live in an apartment with moderately efficient and easy loaded speakers. My power requirements are so small that I use a Teac CR-H220 CD/Receiver for my main system which is rated to provide 25 watts per channel at 6 ohms (1kHz) both channels driven. I've never experienced a shortage of power using this 10 pound piece of equipment and I've never felt that I would benefit from more power for the application (console video games, movies, television, and music listening). Your requirements will be higher because you have a larger room, however your speakers are also more efficient than mine giving way for a margin of error between room size and speaker efficiency. The size of your room outways the efficiency factor, so you still will need more than 25 watts per channel. Even if you had a receiver that could achieve 50 watts per channel RMS for 5 or 7 channels you'd not likely ever notice a problem with power unless there was a limitation of output constricted by the output devices themselves that wouldn't allow any one channel to have output exceeding 50 watts.
Headroom is certainly necessary, and most commonly on requires 2 or 3 channels to be acting on heavy current demands at one time. A receiver gets its power from the power supply and the output sections work indepently taking power that is needed from the power supply. If only 2 channels are running they can use the entire power supply for themselves meaning they will often achieve higher output capability than it would if 5 or 7 channels were running.
