If you do another closed loop and run another Mak4 on it, there will be a nice current, however,
Another Mak 4 and plumbing is about the same cost as the 6080. Also a concern of mine, besides the current the Mak4,s produced, was power consumption.
My 2-Mak4 pumps used 100 watts each and my 3MD used 90 watts, which comes to almost 300 watts.
My pair of Tunze pumps use 18 watts each, or something like that.
Another consideration for the long run is current generators in the tank. You already have powerheads, so the streams, although larger would be no different. Perhaps if you dont like that look though, and while your doing closed loops, all the inside units could be gotten rid of, if you prefer closed loops and the larger pumps on the outside.
Many prefer that option as they dont like power thingys in their tank. Also many larger pumps, like Little Giant and Genex, have an excellent reputation for longevity.
If you were interested in that option, then I would look at direct drive pumps like the Ampmaster or Sequence. They can make some kinda flow.
Hope I have not thrown to much confusion into the mix.
Just trying to bring up all different options for ya.