You say 1 loop per bay, 8" on center, so you mean it goes down and back with 2 tubes in 1-16" bay?
That .5 GPM, is that per tube?
The tubing is attatched to floor or clipped into suspension hangers?
Is that one Taco 11 circ for both floors? Is there a boiler pump? and not that it matters much what is your boiler?
If you have autovents on top of tees, that is not only a lousy way to vent air, with the HV circ you have, depending on where their placed, they can actually introduce air. You should be looking at installing a Spirovent or similar.
1/2" shouldn't be run over 300', I prefer 250' (the shorter allows lower amp pumps, even floor temps and quicker reaction time)
There are many factors involved but the usual design of 1/2 PEX for radiant is around 3/4 GPM per loop.
Because of the loop lengths, that pump may be required and in fact may not be enough, which is poor design and commonly made as a mistake, I pay attention to all energy costs and with proper design a lower energy use circ(s) could have been applied.
150 degree water with only 70-75 degree floors tells me the heat is not transmitting.
With a delta T of 20 (normal design) it appears flow is proper, if it was overpumped is would be less and under it would be more, which still points back to amount of tube and or transfer of heat.
Several pictures of boiler, piping and manifolds will tell the whole story