Will i gain anything even at low boost (for now)

All other factors being equal, I am sure you will see gains even at lower boost in the upper RPM range. Not sure what you would see in the lower RPM's.
I think it would be interesting to see a low boost dyno plot, I imagine it would be a very flat tq curve.

Those that want to see what something like this thing will do should check out JayL's and Leatherfaces old dyno plots. Both had short runner mani's, both lost torque down low and in the midrange, but both had very strong topend power.
And by topend I mean 5500+

I am curious why begi says one would lose power all around with their manifold unless +270ish hp.

The size and shape of plenum, and diameter and length of stacks, I would not compare the BEGi unit to this one, no idea what this mani would yield at lower boost or even at NA.
Biggest benefit of this mani would be with headwork (porting, cams etc.) and high boost, this thing would scream.

I'd buy it if the runners could be easily modified, but they are CNC'ed with complex shaping that would prevent using pipe. It would be easier to just start over.

I think you should put it back in your parts bin and wait for the right buyer and sell it for what it is worth.

Right buyer will come along, if it don't sell at this price I will let it sit.

Who knows, I may have a second Miata by then...

Sale pending!

To a new home she goes, but I am sure she will get properly abused!