You need to determine the model number of the SLX brake caliper as the BR-675 caliper requires a wide-type rotor and pad as well. New BR-7000, 7100 & 7120 SLX calipers use narrow type rotors and pads. The SM-RT81 rotor is a narrow type and only compatible with the latter SLX model numbers. Shimano's Compatibity Chart for disc brakes/rotors. Also note the section "Interference Between Disc Brake Caliper & Rotor.". Significantly it advises one to avoid using an M675 caliper with RT81 rotor.
Whether this is the reason your pads are sticking out is still in question.
Determining the model numbers of your caliper and pads and comparing to the compatibility chart linked above would be a good place to start. The "A" at the end of a model number represents "standard" version of the small part (which is what the "SM" represents). Based on my interpretation of Shimano's published info, the variation in the model number SM-RT81 is related to differing diameters available. These different sizes are denoted SS, S, M, L as a suffix at the end of the rotor model number.
Some other considerations mechanically are that the rotor pistons have been pushed back in after the old pads removed and prior to the new going in. Check the new pads return spring is oriented correctly and that the eyeholes of pads and spring line up.