As I understand it, the fan-out as strings approach the bridge isn't dictated by the width of the nut (particularly when you're talking the modest difference of the 1.5" nut). Rather, the problem is more likely that the standard Mya-Moe fan-out is more extreme than with most other ukes: you'd experience the same problem with a Mya-Moe having a standard nut. String spacing at the bridge is much more variable among makers than at the nut, and sadly, I don't know of a single maker who includes this datum in published specs, despite that it has more relevance to a player than, say, the exact width of the bouts or, as I've just discovered, the advertised nut width.
To check on the variability of fan-out I measured the distance...
In short, the bridge spacing favored by a maker can easily dwarf any upper-neck difference in spacing contributed by the wider nut (in theory, a maximum of 1.5mm at the 12th fret, but in fact this difference may actually be nil), and neither the nut width nor scale seem to correlate with the string spacing at the bridge.
So if you plan to trade in the Mya-Moe for something else, don't be guided by nut width—a wider nut may actually help you on the lower neck—but rather inquire about the string spacing at the saddle, since that's likely the real problem you're having with the Mya-Moe.
I assume you've already considered whether the present action might be too high for you—which is no comment on Mya-Moe's setup, people just differ in what action they need. For barring, I have more problems due to action than to string spacing.