I'm going right over it to take up Crandell's question from 'before'
With the extra diameter, maybe balancing would have been 'improved' sufficiently that, coupled with roller bearings throughout, speeds closer to 65-75 mph might have been intended/hoped for?
The 'first' part of this is that, from a purely technical point of view, the "proven" combination of lightweight rods and careful distribution of reciprocating mass N&W used would have resulted in a higher dependable road speed. It seems clear from the historical material that the Y7 represented what the "admission-balanced" version of the Y6 would have been: a locomotive with a high proportion of weight on drivers capable of N&W's economical fast-freight speed, and reasonable acceleration to that speed from any kind of 'check'.
Personally, I think it would have been desirable to use a cast driver of some kind for the main, but it might be noted that if Glaze's balancing is extrapolated to smaller drivers there is room in even a strengthened spoke main for the comparatively small proportion of balance mass the formula calls for (the vertical component of piston thrust and perhaps some of the reciprocating momentum) in that driver. (I'll say more about cast drivers in a bit.) Rods would be strengthened to accommodate the effect of more mass in the adjacent drivers. On the forward engine in particular, more overbalance than 'usual' for N&W might be required, as the relatively short lever arm of the single-axle lead truck would be poor at providing lateral compliance on the order of that seen on the Js, and it could be argued which of the driver axles should receive more or less of the 'overbalance' to help with augment handling.
I'd expect a Y7 to share the design of the equalization on the A class, limiting the vertical excursion between engines to a minimum (as Alco would do later on the Challengers and call "its" improvement) and it is possible that enhanced methods to prevent nosing of the forward engine could be incorporated there. I'd expect much more damping than 'spring return' force there, if that were to be done.
Now, it might be interesting to contemplate what a locomotive of this type would do if equipped with Timken lightweight rods and bearings sized for the expected loads. I don't think this is as likely as the installation on the As was (because the parts would most probably not be 'common' between the Js, which had all the tooling and know-how amortized, and the Y7) but it's by far the best approach to implementing rollers in the rods, which was the next 'logical' step for easy semi-automated maintenance in fast freight (which I think was the likely justification for the experiment with roller rods on the last A locomotives).
The use of cast drivers might be necessary on a locomotive this heavy and powerful -- the peak load on critical parts of spokes and rim being much more extreme on the design as a whole, let alone its operation at high road speeds on N&W's typical profiles. (It is entirely possible that some variant of Web-Spoke would address this perfectly with minimal additional metal) It would be fun to have seen what N&W decided upon, and then to have watched them 'learn' the secrets of proper disc-driver production... perhaps under license but perhaps not.
Advice of the day: Do not feed trolls, even under other people's bridges.