Thank you, dboomer, for that explanation. I got the 250W off of the spec sheets for the speakers (the L3M is here, for example). I wasn't actually aware of the UL rule for speaker load ratings, but it makes sense. I often here people say "a watt is a watt", and that's true, but it depends on the actual load condition and when that will actually be seen. The acoustic side of dealing with speakers is much more complicated than the electrical side, at least when we're talking about power ratings and all that.
As far as the electrical load side, dboomer is correct too. Using a 15A power strip essentially acts like a bottleneck on the circuit if you're dealing with 20A breakers. If you need the extra 5A, there actually are 20A strips available. Personally, like I mentioned before, I appreciate having the extra circuit protection. The breaker at the board is really designed to protect the wire that's connected to the breaker, not necessarily the equipment downstream. Also, if you do accidentally trip the breaker in the strip, it's a heck a lot easier usually to reset that breaker rather try to find the panelboard in many venues.
For load monitoring, what you would need is ammeter. Ideally, you'd want one that could record peak demand over a certain time period. It would be an interesting little project to hook one up during a show to see the types of draws you get. There are a few meters I've seen that plug into a receptacle, but I think most of those are designed to measure energy, kWh. That really won't help in trying determine the peak loading of the circuit.