Joekster
It doesnt do aga modes i know that for sure, it may not work with a1200 at all dont know.. its a known fault.
As far as power, if you have an expanded amiga with standard psu for sure you will need an upgrade psu. See my spec, I was running it fine with the 4.5amp (best one) a500 psu and when my SuperGen is hooked up i get crashes and mem probs (turns out to be power) now I use a Bigfoot psu and its fine.
Oh and you are correct about it NOT being a Chroma Key, you would need a seperate box for that.