I don't know how to calculate the answer, but we've got radiant heat floors in our master bath and it keeps our entire master suite warmer than the rest of the upstairs. Ours is larger (180 sq ft of radiant heat wire), and has 2 outside walls, so that might make some difference, but it definitely warms the room, not just the floor.

I think the electric heat mats are only designed to heat the floor and not the whole room. They barely draw any power at all - for the 50 sf that we are planning to put in our bathroom it draws a whole 5 amps. I can't believe that would be enough to heat an entire room.

We have a shower-toilet area that's 7x6. The mat probably covers about 4x6 area. We did put in a wall heater to supplement the mats but it has never come on. The mats seem to keep it plenty warm just fine. There are 3 inside walls and the 4th is into the foundation (it's in the basement).

If the one outside wall has a window in it, IMO, you definitely will need a heat source (more than a mat) in the bathroom.

If the outside wall was insulated with spray foam (and no window), I'd be willing to forego supplemental heat.

In between situation? A gamble.

I did that same exact thing (removed radiator, added whisper warm and floors, old house) in my master. It has only been a week, but we have had two major snowstorms (including a nor'easter today) and so far so good. The floors are nice, especially in the middle of the night, but don't really seem to add to the warmth of the room. The whisper warm is awesome and totally heats the room when in use. My master bathroom is same size as yours. We have a tiny transom window though.

Another question, where are your pipes? Ours are all on inside walls, so I am not worried about them as much. If I had pipes running on the outside (hope not in MN!!) then I would want that room heated more evenly in the winter. My bathroom can get cool when not using the fan, the floors don't heat the whole space.

Thanks for everyone's thoughts so far.
Update...went to bathroom showroom to investigate fans. The salesperson thought perhaps Whisperwarm might not be the best solution...but she didn't know for sure. I do have 11 ft ceilings...she thought the heat might mostly stay near ceiling and not warm room as much as I'd like. (?)
I plan to have a window (Anderson transom-type) installed on wall about 6 feet up on shower wall to let in some natural light (window used to be there originally)...unless I change my mind.
The pipes (uninsulated) to 2 radiators upstairs run inside the walls.
I also investigated replacing large old radiator with smaller Burnham Baseray radiators....it's only 2 1/2" deep 10" high could get 1 1/2' to 6'. My furnace guy is coming tomorrow AM so will hopefully get some answers on possibilities from him. I also found Charleston Pro Series Radiators on the web - could maybe get 16" high slender radiator installed on wall. That seems nice - keep that mess off the floor.
Anyone know about either of these? Maybe I should ask on the 'heat' forum?

I'm trying to also keep energy efficiency in mind...don't know if it's wise to remove nice quality steam heat from room that was working just fine...and replace with fan using elelctricity.

I think you might need more heat than would be provided by an electric mat under the floor. If your old radiator tended to overheat the room, the best solution would seem to me to go with a smaller radiator, or a baseboard unit connected into your existing steam heat system. Sounds like you're on the right track looking into the Burnham Baseray radiators.

One other possibility might be a towel warmer/radiator unit. I know these are available for hot water systems, but I'm not sure about steam systems. The combination towel warmer/radiator will provide room heat, but also deliver warm and toasty towels. Believe me, once you've experienced warmed towels....you'll be spoiled forever. Not only are the warm towels a wonderful luxury, but damp and musty towels will be a thing of the past.

My towel warmers are from Runtal. They make both hydronic and electric models. There may be something similar available for steam heat systems, your HVAC contractor will probably be able to advise.

Here is a link that might be useful: Runtal Towel Warmer/Radiators

If you have any electrical outlet or switches on your east wall, remove it and look inside the wall to see if there is any insulation in the wall. If not, you might want to consider putting some in. You might also hang a thermometer on different walls in the bathroom and see how much temperature variation there is.