Well, late news, and good and bad...
Slate is one of the hardest stones to heat, but it's a two edges sword. It takes longer to heat up, but it holds the heat longer. With the variations in thickness, you may feel differing temps during the warmup phase, but once warmed the heat will naturally radiate to all parts of the stone.
If the wires are spaced properly, ( I dont know the specifics of the brand, but Suntouch Warmwire can be spaced at 2" intervals yielding 15 watts per sq. ft., about 51 BTU's/Hr)
Then it should heat O.K.
It should definitely take the chill off, if not provide some additional heat to the room, depending on stone thickness, and heat loss from that particular room.