If the boiler is fully heat up, and that you indeed kill them right as they spawn (so max spawn rate for the spawner), them you might be able to keep your boiler heat up.
The problem being that for way less work, a 1 chunk tree farm can also do that, be it with mfr or railcraft.
Blaze rods used to be really good for boilers, but now they are just not worth the trouble. Look at the heat value for them, and compare it to planks, charcoal, or any other readily available solid fuel sources, and you will notice that it's been nerfed so hard that even your T5 will have trouble keeping up.
The problem being that for way less work, a 1 chunk tree farm can also do that, be it with mfr or railcraft.
Blaze rods used to be really good for boilers, but now they are just not worth the trouble. Look at the heat value for them, and compare it to planks, charcoal, or any other readily available solid fuel sources, and you will notice that it's been nerfed so hard that even your T5 will have trouble keeping up.