Depending on where you live, the building code may require that electrical heating be installed on a timer with the maximum on-time of 30-minutes! CA is one of those areas. FWIW, most systems are designated as floor WARMING systems, and not room heating. Depending on where you live and the heating requirement of the area, it MIGHT be enough, but would again, be problematic depending on where you live, since you couldn't legally, leave it on for room heating.
To be useful under those circumstances, you'd want the heating wires as close to the bottom of the tile as possible. Depending on the system you choose, and if you embed it in SLC, you might have a significant thermal mass, and 30-minutes may not do much!
Schluter's system - DitraHeat overcomes some of those issues. It CAN be installed in a shower, but only if done exactly as they say in their installation instructions (basically, you'd need to make a Kerdi Shower - check their website at
www.schluter.com). Because DitraHeat doesn't have as much thermal mass because of its structure, and that structure has a matrix of air channels, the heat tends to be distributed more evenly and faster than other techniques.
FWIW, while use of an ohmmeter is important to verify you have not broken a heating mat or wire system, it won't check that you've damaged the insulation which can lead to corrosion and failure, often at least several years down the road. As a result, Schluter requires that you use a different tool - a megohmmeter. This device not only verifies no shorts or opens, it also tests the insulation. If you don't, the warranty is void. Most brands, in the fine print, suggest you do this, but do not require it. Schluter is very conservative and wants it to work long-term, so they require it.
Most heating mats/wires can produce at the max about 12-14W/sqft, which equates to about 40-48 BTU/sqft. Now, note, that depending on the room, you might not be able to get those maximums, as there's a limit on how hot you can safely make the floor.