The problem is, though the DR system divides things into different tiers, it doesn't explicitly state that it is forbidden to mount more powerful weapons on them. Especially if In-Character, it is possible to do so. Additionally, even if we allowed fighters to mount SDR level weapons, nothing would particularly change IC either so long as the proper rules are made to prevent that from occurring. Of which, there are none written down on the wiki anywhere as far as we can tell.
But even then, with this submission in specific?
What is the difference between an SDR 2 weapon and having two ADR 5/SDR 1 weapons instead? As far as I can tell, nothing other than interactions with shield threshold, to which I suggested a fix as well. So, why restrict this, especially if the craft is psychically capable of mounting just one SDR 2 weapon rather than two ADR 5/SDR 1 guns?