The message from snipped-for-privacy@care2.com contains these words:
You are sounding more and more like Dribble every day. The difference in efficiency is only marginal. The larger thermal capacity may even be an advantage in a boiler that doesn't modulate. You even played the typical Dribble trick of inserting an additional qualification to bolster a weak argument. Take out the "a given physical boiler size" and you can match the surface area.
What could make a significant difference but which you fail to mention is the difference in thermal conductivity between Aluminium and cast iron but there again that is more a question of size constraints than any inherent unsuitability of cast iron.
Now why would the op be disappointed to find that the boiler in question has say a 70% efficiency rather than the 50ish% that you said was his lot.