Hello. My name is Mark and I will be assisting you today.
I don't think replacing the exhaust is the answer.
If the exhaust was blocked then it would be worse at high revs as there is more gas to remove.
If it is under powered at high revs and also back firing at low revs then that could be a blockage.
Your question isn't very clear about what is happening at high revs.
Is high revs ok with no problem or is high rev ok but problem starting to occur at higher revs also?
That sounds like you have a burnt valve or two.
First thing to check is cylinder compressions and then a cylinder leakage test.