In addition to maintaining access to the researched building and hull types, you also need to maintain labs to *fire* your superwearpons, even if you already built them.

There is really only one case in which I scuttle labs, and that is when I built two civic labs to research ice and/or volcanic colonization.  These technologies, and a few others at the lower tech levels, remain accessible after the labs are scuttled, letting me continue to colonize these worlds while putting all my logistics space towards factories and military labs.

Otherwise, later on, you should keep all your labs operational.  If logistics space is actually so important that you need to consider this, your empire is probably so small that you're not even ready for 8 civic labs yet anyways.

The point is, for an empire that small those six labs are not worthwhile.  You're better off saving the space for military labs and expanding your empire.  If you've got a bigger empire, Development Mandate is positively awesome, but if you're so small you're struggling to fit in all those labs in the first place, mandate on its own won't solve the issue.

I'm a firm believer that the ONLY real solution to logistics shortages is to expand your empire.  Six labs may sound manageable from a logistics standpoint, but that's a lot of cost to pay when most of those technologies are not viable for medium to small empires.  At the end of the day, you'd be better off sticking with 3 civic labs and saving those logistics slots for the extra military labs you do need.  This solves your logistics problem much earlier and cheaper.