A couple of things may set the cat amongst here , fuel load and exhaust muffling . Long range tanks are bulkier as are mufflers that comply with NSW standards anyway .
Fuel load shouldn't be as big a problem as with the FDL-16...
The 12V250 (GEVO-12 equivalent) weighs 20.19 tonnes dry
The 16V228 (FDL-16 equivalent) weighs 22.14 tonnes dry
and for comparison
the ME16G7C (16-710 G3C equivalent) weighs 18.37 tonnes dry.
So based on the engine weight you have nearly 2 tonnes more fuel available.
The big question is the cooling. The huge radiators and intercooler on the ES44DCi lengthens the locomotive by one full metre compared to the domestic models. While South Africa isn't the Pilbara, I understand it still gets hot there. Width may not be a problem but fitting everything into thirteen feet high might be...
I wonder if the reduction in power to 4000 from 4400 HP is driven by the need to limit the size of the cooling package? There shouldn't be a problem with putting 4000HP to rail using AC motors, even on 1067 gauge.
The PH37 MAi was designed to meet 120 tonnes gross with limited fuel. Clyde/EDI/Downer wouldn't have sold as many GT42CU AC units if they had weighed 132 tonnes. They wouldn't be working in the WA South West if they weighed that much and some lines in Queensland would not be available (PN wouldn't be able to use them on intermodal trains, for example).
With the reduction to 4000 HP for whatever reason, the new locomotive is not as much an improvement on the PH37 (300 HP rather than 700HP).
If the power reduction applied on standard gauge locomotives in Australia, why would anyone want a GEVO instead of the FDL which already provides 4350 HP, even if the engine weighs just under two tonnes more?