Baking Sirloin in Oven on Pizza?
I would think that if you were using fresh sirloin and the pieces were small enough you would be alright using the method you mentioned. I would think that if you were to pre-cook your sirloin and then cook it on top of the pizza for that long you might get tough sirloin. I would suggest cooking the sirloin until just about done, while the pizza is cooking then putting the sirloin on the pizza (I would add a little more cheese at this point- but that's just me), and giving it 2-3 minutes under the broiler to melt the cheese and cook the sirloin the rest of the way. 
To avoid losing the flavor of the meat, you should marinade it for at least 30 minutes and season well before you precook or put on your pizza(use a meat tenderizer in your marinade to avoid tough meat- vinegar or pineapple juice works well.) Good Luck- I hope it turns out delicious.