I agree with JamesM in that if you have a 58mm tamper, a fitted one isnt going to solve your problems. If it were the typical 7+mm undersized piece of plastic that comes with a lot of machines, I would say you need a new tamper. But I dont think that is the case for you.
Play around with variables and techniques a bit more as JamesM states. They do need some playing with to get optimal results, but Im very happy with what I get from mine now.
I do have a fitted tamper, because I knew I wanted a Pullman, so why not fitted? But I dont think its necessary.