The question you should be asking yourself is: how does X weapon compare to an MLB disaster assault weapon? In general, an SSR weapon stat-wise = SR weapon + 5 levels. Therefore a LV85 assault SR > LV75 assault SSR. Sure, an SSR weapon has a +3% skill ceiling, but you pay heavily for it. So in general, for weapon skill purposes I wouldn't invest in an SSR assault weapon without at least 2 LBs, and definitely not a defender weapon that wasn't MLB. Of course, if it's a weapon that can eventually be MLB'd via a renewal, then that changes the cost/benefit analysis.

Something to note is that assault SSRs are precious (going by my Water schedule). After a year of play you'll likely have 3 or less, so you really don't want to miss an opportunity to MLB any of them.