the plot thickens A15E dual outlet manifold, never seen it DD
is there a pic of that?

As for exhaust heat vs water circulation, one would get hotter
quicker than the other and much hotter overall indeed.

As a purist or looking for good all round benefits to your kb10
eg. good economy, smooth running and factory performance
if you are using kb10 A10 or stock A12 with stock carb or slightly
upgraded induction, the kb10 manifold is a good choice but stock
manifolds flow enough for all but 90+hp capable A series.
Exhaust flow of stock mani do not restrict these little engines.

My slightly higher compression A12 with stock everything
except upgraded dizzie & kb10 A10 re-jetted carb was simply magic.
I also used my stock A10 flywheel with higher rated clutch.
This simple setup out-accelerated a similar A15 engined vb10
and he sported headers, weber 32/36, electronic ignition etc.
The smaller A series are good little rev happy engines.