I previously owned a Mango for 15 years with the optional tank (overall capacity was 1000l) and I often used the 2nd tank capacity.
When I went for Atlantic and Pacific crossing with my SM, I was of course very skeptical about the 600 l of her tank and like you I've placed on the deck 6 Jerrycans (which I didn't use during the passages (but gave me a great peace of mind ;-)
The absolute difference between a Maramu/Mango and a SM is that with the ancient designs you have to use a lot more the engine in light winds which is often the condition for passages. FMHO you're both right, additional Diesel is a necessity for passages with Maramu/Mango designs and can be avoided with SM
[Note] Additionaly, the SM has got a generator when you have to run the Perkins for energy with the Maramu/Mango and it also make a difference in consumption.
I hope it helps
S/V ScentStone SM2K #375