GE's AC6000CWs had their share of "issues" in the beginning, but as far as I know, the traction motors were not among those issues.
CSX has re-engined many of their AC6000CWs with GEVO-16 engines rated at 6000 HP, and they seem to be performing fine based on what I've heard.
The only issue I can think of in regards to emissions is that you'd need a fairly massive cooling system. Look at the radiators on an SD70ACe... they're the same size as the radiators on an SD90MAC-H. The radiators on an SD70MAC are much smaller; EMD had to make them bigger in the SD70ACe in order to meet Tier 2. It stands to reason, therefore, that if EMD or GE wanted to build a Tier 2 or Tier 3 compliant 6000 HP locomotive, they'd have to make the radiators even bigger. And when Tier 4 comes into play, one or both builders may need to add additional equipment such as particulate filters or urea injection. Finding space for that stuff in an SD70ACe or ES44AC will be a challenge. It would be an even bigger challenge if you tried to do it to a bigger locomotive, without making the locomotive longer or taller or both.
Pan Am Railways — Boston & Maine/Maine Central — Delaware & Hudson
Central Maine & Quebec/Montreal, Maine & Atlantic/Bangor & Aroostook
Providence & Worcester — New England — GE Locomotives