quite simple really
the wb is at the 'limit' of its effectiveness
which is to say that even a lot more flow gains very little temp reduction
and yet again
what do you think the initial flow rate was ?
and after the pump change ?
use the pump P-Q curves for a fair estimate,
make a simple manometer across the pump's inlet and outlet for more accuracy
if you can measure that gain . . . .
of course the worse the wb is, the more change one will see
an observation:
we are now at the stage where this stuff is known, and has been demonstrated;
perhaps its time to start applying the info to the system's design ?
better components are not needed, more thought in their selection and compatability is