With a basic porting (A little widening of the ports, clean up and removing the base gasket) I agree with the 60%. Fixing the cat muffler alone is good for 30-35%, then gaining 20-25% on the motor would would certainly put it at about a 60% improvement.
To cut the times in half it can be done, but the limits will need to be pushed a bit more. Work on the carb, piston, and another 20 psi of compression and an 8 tooth gear. Then it would be cutting in half the time. Team that up with a good race chain and it should be down to under 3 seconds. Modified, but still be good cut fire wood.
Crossing the line to a stock appearing gas race saw it should be under 2 seconds in that sized hardwood wood. put a pipe and some fuel in it and I would not be surprised if it cut close to 1 second.
What kind of ash we talking about? I'v got some white ash that is near as hard and dense as red oak, and some other green or black swamp ash that about as easy to cut as poplar.