Well, I bit the bullet and installed MT couplers on 11 modern Kato locomotives (ES44, SD70ACe, AC4400), and here is what I found:
2004 couplers: these offset couplers, designed for low-riding cars, and just too high when used on the newer Katos. The MT Conversion chart calls for 2004 couplers on many older Kato locomotives, but in my hands these ride way too high on the newer models, with the result being sporadic uncoupling events, especially when the train passes over a kink in the rails. The MT conversion chart is out of date, which is odd given how common the new Katos are.
1015 couplers: I assembled these using the MT jig - not bad at all once you get the hang of it, which does not take long. However, the coupler box is larger than that on the 2004 couplers. I had to use a triangular file to remove the 'ears' (see photo), but I also had to do some additional filing to make the opening wider and also a bit taller. When I could finally fit the 1015 in the opening, the Kato clips that normally hold the couplers in place sometimes worked and sometimes didn't: the issue is that the 1015 box is thicker (in the vertical dimension) than the 2004, so sometimes the clips just don't snap in to place. However, this issue can be solved by simply using the metal screw provided with the MT couplers and discarding the clips.
A hybrid approach: to avoid excessive filing, I opened pre-assembled 2004 coupler boxes, saved the 2004 box while stashing the couplers elsewhere, and then installed 1015 couplers into the 2014 box. To fasten the box lid back in place (it won't snap in place), I held it in place with tweezers and then touched either side with a hot soldering iron. This worked great, though I had to do some very light filing to remove any flash that extended out as a result of melting.
The 2004/1015 coupler hybrids fit into the openings after only filing the ears (photos), and the Kato clips then held everything in place. In the end, I found this approach to be less work that using 1015s. Of course, this does result in some wasted materials (empty 1015 boxes and 2004 couplers), but perhaps I can use these on some low riding freight cars where a straight 1015 would ride too low. Also, purchasing 10 unassembled 1015 couplers is pretty inexpensive, so I can live with it.
As an aside, the Kato couplers have a little brass sheet behind them that functions like a spring. Just discard this when installing MT couplers since these have their own centering springs.
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