There is really no correlation between sound and power, just go by the feel when you stomp on it, ie. where the dead spots are. Do you have the dyno printout? Live readings of fuel flow etc would be very useful.
I don't know what typical timing is for those, but 10 strikes me as way too low. I'm thinking at least 15 to 20, but I'm not sure.
Nothing wrong with downdraft webers. I don't have practical experience so perhaps someone will step in, but my impression is that they can be great for performance but are more finicky than others and need to be precisely tuned to live up to their potential. You get what you pay for type of thing, great if tuned right, otherwise a nightmare (in other words it doesn't "just work") The most negative things I've heard all have to do with altitude sensitivity. They also have to be tuned to each other. On my old SUs someone just flew by the seat of his pants and got the engine running seemingly normal by messing with everything independently, the result being that one carb was set way too rich and on the other you basically couldn't get it off choke. Miraculously the engine sounded fine, but as expected, no real power.