Kingsley, do
you think having two 303s (from /entity/Q$1 to
/wiki/Special:EntityData/$1 and another one to wiki/Special:EntityData/$1.xxx)
would be appropriate or at least better than what we have now?
Yes.
303 is what you want. Also note that 303's can now be cached re. newer HTTP spec
guidelines. Thus, this is your simplest fix.