Boosted doesn't increase power at the output. That depends strictly on the power supply of the amp. Boosting increases the distortion or the amount of frequencies in the sound.
Most likely, the reason you hear the Marshall over the Mesa is that the Marshall has most of it's energy in a tighter frequency band, while the Mesa is smeared across more frequencies. (This is why distorted amps are much harder to cut through the mix than clean amps.) Your ear only picks up the loudest at a given fequency - also why much of the bass and low mids disappear live as they're covered up by the bass and the snare/toms which are louder at those frequencies.
As noted, those old Marshall were rated at 100W, but that's an average power rating and they also rated those like hi-fi amps, so with low THD compared to where anyone ever plays them for guitar.
In the 50W vs. 100W Marshall, I definitely always preferred the 100W versions due to the tighter low end and bigger bass, but the difference is fairly slim and I could happily play the classic Marshall 50W too.