You keep mentioning conversions, but that's far from a benchmark for what a machine capable of, especially in the early 80's. Much like the Amiga games designed for the machine show what it's capable of more often than badly coded conversions. Gimme Katakis, iO, Armalyte, Citadel, Cybernoid, and a bunch of others over the arcade conversions anyday. Also, just because conversions are poor doesnt mean the machine isnt up to the task (hmm,... this is familiar). Unlike some of the systems you've mentioned developers of c64 ports seldom received the same sort of help that was given to big name Asian compaines. Even if it's not (up to a particular task) so freaking what ?
End of the day the c64 had some great shooters, regardless of what other machines did better/worse.