We are based in CT where it can get very cold in winter.

The electric floor heating in my experience will not add much warmth to the room. They are intended to warm the tiles for foot comfort.

I live in Iowa and last week our furnace went out. We were 5 days without forced air heat propane burner type. In the bathroom I am remodeling, I kept the radiant floor heater on, cycling from 72 to 84 (factory preset, I haven't adjusted yet) and with the doors closed it maintained a room temp of 56. My system is 240v wire that SunTouch makes, called WarmWire. I have done this twice now. I have not used the matts.

The bathroom is about 575 cubic square feet, 72 sf floor, and 38sf warming wires. Room is 8x9'. I did not want my pipes freezing because during this time we had a cold snap (again). Other parts of the house were 46 degrees. We used space heater for warmth in 2 rooms and could maintain about 62. Now got a brand new furnace:)

Our experience with electric heated floors in bathrooms are that they do make the room warmer. Recently we were wondering why one of the bathrooms was so hot all the time during an extra cold winter. Turns out someone hit override and set the floor temp at 80 degrees 24/7. Once we reset it, it is more comfortable. I always notice the difference in the bathroom temp when I raise or lower the floor temp. Both bathrooms also have forced air heat.

Is the bathroom currently comfortable on the coldest days (outside of a cold floor since you don't have a heated floor)?

Assuming it is and you want to replace the baseboard with an equivalent heat source, you'll have to figure out what brand of baseboard you have and what its rating is per foot for a given boiler water temp.

For example, if you follow the link below for Weil-Mclain baseboard, you will see it's rated at 565 BTU/hour/foot at 180 degree water temp and 1 GPM flow rate. That gives you 3390 BTU/hour for a 6 foot element. Subtract the 1520 from the Runtal and you're 1870 BTU/hour short. Figure out what your heated floor will deliver and you'll know if you're covered. The floor control would have to operate in air temp mode, not floor temp mode if you expect it to heat up the room.

Here is a link that might be useful: Weil-Mclain baseboard rating

Very basic reply based on a very generic rule of thumb.

For generic construction, you can figure a WAG of 30-35BTU/Hr/ft needed in your bath. Again, that's a generic WAG. If your house is older and the bathroom has exterior walls, windows, a poorly insulated crawlspace below and a cold attic above, you may need more. If your house it tight and well insulated, then you may need less. Thus, the 30-35BTU WAG.

You say the bb is 1520BTU, for your 100sqft bathroom that equates to 15BTU. Most floor mats give off roughly 12-15BTU on average. Check the specs and you might find one with a higher rating. So if you add 100sqt of 15BTU mat to your bathroom, then you'll be at about 15.2 + 15 = 30.2BTU/Hr/sqft.

That might be sufficient, but that's also using the output of the RFH mat as an actual heat source versus simply a source of under-foot comfort. And it's also using both sources, the BB and the RFH mat, at 100% of their rated outputs.

If the RFH gets overstressed, CL&P may send you a thank you card along with your monthly bill.

Again...a WAG.

In my house I have Verano's system and it is the main heat source there.

We used Warmup for our family room/kitchen. I seem to recall that they were rated for sole source heat in some climates. Check out Warm Up.