If I understand correctly you're wondering about the affect of the aircraft accelerating on the exhaust gasses which are travelling forward from the exhaust port? If you've got a 1:1 thrust/weight then it'll accelerate at 1G horizontally (until drag builds up) so there's a 1G force the exhaust has to work against. Now given that all the exhaust has to exit the cylinder in roughly 1/1000 of a second (depending on revs and the time the exhaust port is open) then I'd say your power loss would be on the order of 0.000000000001%