I live in your neck of the woods (just outside of Philly) in a 2400 ft^2 home. It is a split level. Thermostat on the 3rd floor. Middle floor drops a decent amount (8 stairs) and first floor (3 more stairs) on slab, built in 1964. Sound familiar? Insulation is crap. Added R60 blown in cellulose to the attic after complete clean out and air sealing though. With that done, I put an insert into the fireplace on the first floor slab. Regency 2450hi is the model. The thing makes massive heat. My IR gun shows surface top temps (STT) of 800 regularly on a full load with the air control fully shut. If I jam it full of wood I can get a good 8 hours of burn from 800 cruising slowly down to 400 when it's 25 degrees outside. Yet, on a sub-20 degree day, I have not been able to get that lower room above 66-degrees (portable thermostat placed on the opposite side of the room, 16 feet from the insert.) Today, high of 46-degrees and sitting in the room right now with that thermostat reading 73-degrees. Noticed you said that lower room is 45-48 degrees all the time. Sounds like mine, but worse. Mine is 57-degrees all the time when it is 25-45 degrees outside while my furnace (forced hot air) is running according to that thermometer on the 3rd floor set at 68-degrees. Unfortunatly, my 2nd level (the biggest open floor plan space) is just as cold. I wake up each morning after that furnace has been running all night and its 57 degrees on that 1st floor. BAH. (I only burn my insert from 4pm - 12am because it is not efficient enough so I'd need to load it full 3-4x per 24 hour period but not really interested in that.)
I say all this because your situation seems similar. I tell you I was very disappointed when I put in the insert and couldn't get the room to become like a sauna. I want 85-degrees in there. I work hard to get my wood all bucked nice, split, stacked, tracked, categorized... and then seems like not enough. I believe it is a combination of (1) insert losing too much heat up the chimney & (2) bad insulation. My chimney is on the side of the house, so, exterior as they call it. It's outside the envelope of the house. Not so good for keeping that heat in.
I am about to crush my lack of heat by putting a Blaze King 40 (which is a free-standing stove) in my big open floor plan 2nd level because that's where my family really wants to spend most of their time. That's my solution because adding insulation ain't gonna be cheap, and this will be way more fun. Also, all that insulation... I did the attic but now there's the freezing cold floor of the 2nd level, all the walls, the windows. yuck. I'm planning to produce a lot of heat, then perhaps work on more insulation to keep it in. First order of business is to produce enough heat tho...
You also say "less time throwing wood in"...yes thus the attraction of the BK40. I am planning for the same reason.
So I recommend you look seriously into a free-standing stove because I haven't been able to get enough result from my insert. Keep that heat INSIDE your home!