Regardless of the gear ratio, the truck/suspension design itself has limitations. "Standard" switcher type trucks that only have primary/vertical suspension are typically restricted to 45 mph, and I have heard that B Flexicoils were limited to 65 mph presumably the limits of its effective suspension design. Transition (or the lack thereof) can also limit the top speed. Dead in consist or dead in transit might allow higher speeds.

By 2300s I presume you are referring to the SW1200s. These should have been in full series when at low speeds or when kicking. I've heard claims that this allowed them to pull better at low speeds and kick. IIRC EMD had the SW900s in series-parallel when kicking, but full series to series-parallel in road.

The SW1500s (and most SW1000s) and MP15DCs are in permanent series-parallel which simplifies the high voltage system by eliminating traction motor connection change transition, with optional traction motor field shunt at higher speeds. This is due to the use of the D32 main generator except for some SW1000s with the smaller D25. By comparison, the GP15-1 using the same engine/D32 combination uses series-parallel to parallel transition presumably to get more useable power in the higher speed ranges.

The SW1000/Sw1500/MP15DC had the Switching Selector Switch allowing them to load faster in Switching 1, and already be in Run 3 engine speed in Switching 2. I have found that these circuits have not been maintained.

Don't know what an LE refers to. I did here that the FM units were rumored to load fast. This was recreated at OERM several years ago and proven to be true. An experienced SP/UP conductor didn't even try as the steps flew by. Not sure if a GE 70 ton could do that, but I can say from experience they will get up and go, and I have kicked cars with them.

As far as GenSets go - I like the concept but think it has been very poorly implemented with the insane focus on fuel conservation and emissions reduction taking priority over reliability, maintainability and useability.