40 thousandths is only .040 (40/1000=4/100=.04) thats very little. I would highly doubt it will be beneficial power-wise, but I am no engine builder. I could do some math and figure out how many liters or cu. in. that would net you, but I really don't feel like it. Others may have something else to say (like Glen) but personally, unless you have a really messed up block, I would not bother. Just bore it out enough to get smooth cylinders, then call it good. I think that is all boring an engine out is really for, to get a good seal when putting in new piston rings and oil deal (I forgot what that one was called). If you are looking for power, get a header, and a more aggressive cam, and gasket match all the intake (I dunno about the exhaust ) and hope for the best. Hope this helps you out!