On ultra low sulphur diesel, the two locomotives tested in this government test were the BNSF 7736 (ES44DC) and UP 8354 (SD70ACe). The fuel economy gap you suggest, GP40-2, isn't as large as you might think. Remember that the GE press release that says the GEVO is 6% better than the 16-710G3C-T2 engine is put out by the company that produces and sells that GEVO engine, so you need to dig somewhere else for the REAL truth.
BSFC between the two above mention engines are as follows:
BNSF 7736 BSFC 0.370 lbm/hp-hr
UP 8354 BSFC 0.372 lbm/hp-hr
So while the GE does win in the above numbers, it ain't by 6%.
http://www.regulations.gov/fdmspublic/ContentViewer?objectId=09000064803aa661&disposition=attachment&contentType=pdf
Also keep in mind that the current incarnation of the 710 engine (remember it's a two stroke), is 3% BETTER than that of the old 7FDL16 used in the AC4400CW.