I have had several pieces modded / upgraded / and custom built. In all cases, there were definite improvements and I have been very satisfied.
Except, for one issue -- it has been my experience that several modders / custom builders have frustratingly long, long, long turnover times. I've waited 6 months one time for a CD player clock to be swapped with a new one and I'm currently looking well into the 7th month of waiting for a custom build.
I know, there is a multitude of factors (one man shop, supplier issues, etc) but dog-on, the state just finished a bridge rebuild were I live in 4 months (with the lowest bidder)...