Useful rule of thumb.....when we were designing the Matamp varipower, one thing that became apparent in real world scenarios is that you need to work in factors of 10.
What I mean is, to be twice as loud as a 1w RMS amp you need a 10w RMS amp, to get twice as loud as that 10w amp you're going to need 100w. And to get twice the perceived volume of a 100w amp you need to throw 1000w at it.......works very well for PAs too. We did this completely unscientifically, at gigs and rehearsals, and infront of a room of players in a blind fold test....
So 550 is only twice as loud as 55w!! But as Peterku says, the extra headroom is the most important factor. For powerful grinding metal and clean cleans...you need bags of power - this is the idea behind the 5150, let the preamp create the tone and the poweramp drive the speakers. Lower wattage amps tend to sound softer, and the bass compresses first, so for powerful bass get more watts!
