Frankly, I've had really good results buying older Katos on Ebay, particularly as parts spares. They seem to hold up pretty well as used units, and if one has been weathered or otherwise abused, can go relatively low in price. With a six-axle unit you'd get another 12 wheels.
I don't think you'll find any interest from Kato in supplying higher-flange wheels, even if you're 100% right. I've got a pretty big fleet of C30 and SD40/SD45 chassis (pre-DCC) and I know what you mean. Where I see it is entering and exiting superelevated curves, where there isn't enough lateral motion on the trucks to keep one set of treads from lifting. Doesn't take much to start an outside rail climb. The worst I've seen on my layout are the BLI PA1's, those were impossible to keep on the rails - even my Hallmark 4-8-4 with 4-axle tender trucks has no problem in areas where that thing would pile up. That 4-8-4 is my test dummy, if that makes it through, there's no excuse for anything else derailing, lifting, or warping wheels off.
If anything, I've discovered that doing things 'right' like superelevated curves can make it a whole lot worse with lo-pros on six-axle diesels.
What you need to know, and I don't, is if the diameter of the 'new' split-axle axle with the smaller flanges is the same as the old one so that they really are interchangeable.
