Insulation heat-loss improvement, system sizing

[American Locomotive]

Family member is looking to replace their AC system with a heatpump to augment their oil heat. While I was up in the uninsulated attic looking at the unit, I noted that there was only about 3" of sad fiberglass insulation on the ceiling joists, which is roughly what, R12? Looks like the recommended insulation area for this area is now R60. So we're going to pump that insulation up to R60 equivalent.

The current system is a ~25 year old R22 3-Ton unit, and they tell me it just about keeps up on the hottest days. It's a single story home, about 1600 sq-ft. New metal roof.

So with the insulation improvement, would keep the existing sizing of 3 ton, or would you be comfortable downsizing to 2.5 ton?

 

[The Cobbler]

with similar insulation My original a/c would run all day long & well into the night . I upgraded the insulation to ( probably r60) and based on recommendation from installer( friends Nephew inlaw) replaced the A/C with the same size, ( I think 1-1/2 ton)
it cycles frequently even on the hottest days . I don't have any humidity issues that would be seen from too large a unit . the house id very comfortable
I know that's probably not answering your question, but hope to give you some insight to savings
the summer I ran the new unit, my electric bill was half of the previous year , as well as the gas bill for heat in winter
I insulated the basement walls at the same time