I have a pitts powered by a brison 2.4 and I made no mods to the firewall. I did the mods to the top wing center and I also installed blind nuts on the fuse for the ca bane. The plane balanced perfect with no extra weight, I have no complaints about the landing speed, I had to land my extra faster. My plane weighs 15.5 lbs and has plenty of vert. it is not unlimited but the real pitts isn't either. If you take your time and check it all out as you assemble it you shouldn't have any problems. I cant say you wont have any cause every plane is different. I say just check it over and look at everything real good and make the mods that are needed and go have fun with it.
This is a great plane in my opinion, but this is just my 2 cents
All have a great holiday and a better new year
Fred