I think the ones on the ISS are about 20% efficiency, and currently the record is about 46%, so thats true. Your point about size is true, but they can be made rather compact, depending on the array design. Combine that with the advent of transparent solar cells...
9 miles a second is an issue if you arent also going about 9 miles a second. If there are two ships going 9 miles a second in opposite directions then you should just throw some trash or screws in its path, it would just explode. My rebuttal to the small size thing also included radiators as a target, which are, again, quite large and quite thin, which makes it fun for added heat getting thrown at it. I am not saying that it is better to use a weapon that can only take out sensitive things, I am saying that a small recoil-less weapon with few moving parts is better in a gravity less vacuum than any propellent based kinetic weapon, atleast if you want it small enough for a single operator in a suit and with minimal mass. Laser powercells would probably have far less mass than an equivalent of propellent based kinetic rounds, and mass is a thing that anyone would have to worry about when in space.
Yes, they do sound fun, dont they? The "classic" space dogfight also sounds like a colossal waste of propellent.
See argument about retaliative velocities. Also add to the fact that too great a difference in velocity means that you would go to different areas very fast, also, 9 miles a second in relation to what? Around an orbital body? If that is the case, than you are going to be falling rather fast. Also, again, computers. Even with a slide rule it would be rather easy to plan shots from one orbiting body to another, such as two ships orbiting a planet at different orbital velocities, inclinations, eccentricities...
-ninja edit-
Shielding is, again, rather hard to do on radiators, which are quite possibly one of the weakest parts of a spacecraft (theoretically at least) due to the fact that you want as much heat radiation as possible for the least mass. Which normally means rather thin things. Which means that the particles carried by a particle beam weapon (which includes lasers, albeit they arent the best examples) would typically be able to damage them quite easily.