Since you are used to flying an IMAC type 40% bird I would think that for you to be happy with that 4.2 in the P-51 it would have to weigh no more then 30 lbs. I have fallen into this myself, people give me very strange looks when I tell them my .60 powered 7 lb Super Kaos is a dog. But that is what happens when you get used to flying a 38-40 lb airplane with 90 lb of thrust.