I think I can answer your 2nd question: No. basically, the 3ton unit can be set (the blower) to a lower speed for the 2.5 coil.

I could look, but are these the Xn15 units being spec'ed?

The trane outside units are both XL15i.

Cisco

there are three equally important components-quality HVAC, the install by dealer, and probably the most overlooked and disregarded is the ductwork system.

these are my minimum specs for a new HP system. both outside and inside units should be replaced to have a properly matched system.

15 SEER, 12.5+ EER, 9 HSPF
best matching VS air handler
full BTUs in both cooling and heating for your rated size
R-410a refrigerant(same as Puron)
scroll compressor preferred
electronic demand defrost preferred
thermostat with "dehumidify on demand" feature
staged backup heat strips
new and correctly sized refrigerant lineset

you want a thorough inspection of your ductwork system. size, overall condition, supply and return lines, insulation qualities, leak test, etc.

any hot/cold spot issues in your home should be addressed.

I would only use authorized dealers for the various brands that provide quotes. see mfg websites.

I would look at Trane/AmStd,Rheem/Rudd,Carrier/Bryant.

Depending on your location, I would not purchase a new HP system that did not have electronic demand defrost.

IMO