Wensleydale: good point on systems integration.
That doesn't answer the mail on the point under consideration of
airworthiness. An aircraft may be perfectly safe to fly (airworthy) and the kit won't work quite right (mission degradation) ... or the add-on kit has some problems ... no few aircraft systems have run into that sort of problem. While not meaning to bore you with helicopter stories, I know a fellow who had some uncommanded flare/chaffe launches during dynamic interface testing some years ago ... it was a surprise fireworks display! The aircraft was airworthy, but the add-on kit needed a bit more work to be properly integrated into the overall air platform. I flew some airworthy Seahawks whose mission was degraded for a while due to some systemic problems in the blade de-ice system. Again, airworthy, but full mission potential not realized due to a particular (and systemic) problem with that sub system.