I have a Yosh TRS on the stock header. I was considering trying to find a Yosh 4 into 1 header, but I was wondering how much more power I can expect over the stock 4 into 1 and the general price I would be looking @ for just the header.

Well in general a slip on gives you about a whole fat 1 or 2 hp max at top end. Sad but true.

The full system will give you from 4-7hp depending on the rpm. And yes when the fuel system is mapped right you can feel the difference. I have one with a slip on and one with a full system 118 hp on one and 123.5 on the other.