600 rms is probably a good guess but I wouldn't surprised to see a bit more. It has a fairly stout power supply for such a little amp. I have seen some 1k class d amps with a similar power supply in them but of course this won't be quite as efficient. The fuse rating on the zed amps isn't always a good measure of the amps true power. There used to be a good article on the Zed Audio site explaining their take on fusing. The way zed does it they offer a lot more protection for the amp while still fusing high enough to handle the in rushes of current you would see while actually listening to music(dynamics) . A lot of other manufacturers fuse for the maximum current draw of the amp which offers less protection. Remember that a fuse can handle much more than its amperage rating for short periods of time. The Zed Audio built US Acoustics USX800F came from the factory with only 60 amps of fusing but would put out over 1K. It was built a little tougher than this amp here though. Wouldn't be surprised at all to see 800 watts out of it.
As for the switch I imagine all it does is change the rail voltage in the amp to optimize it for either 1 ohm or 2 ohm loads. I doubt it offers more power. Just increased flexibility. You can change speakers/loads without needing to buy a new amp. Great pick up for what you paid even if you quadrupled it.