Well about the lenses, you will not gain anything in sharpness with LF lenses. The Mamaya lenses are very sharp (as you already seem to know). The pros of LF lenses is to use them with LF cameras with tilt and swing and shift... i.e. the large image circles which allows you the use the movements available with the camera and lens combination. Also, if you go for older, not that modern glass, you will find out more about the way LF lenses (on LF film i.e.) have a "soft sharpness" to them.
Another way of seeing it is to know the way you want to work. The Mamaya 7 (like the 6), is somewhat like a Leica and thus quite fast to work with. Possibly a tripod, but that's it. A Fotoman or to extend the idea, a fully fledged LF camera, will on the other hand need maybe 10 minutes (or even much more) to set up the shot. If the idea of slowing down that much intreagues (sp?) you, then go ahead and enter the LF world. Else, the Mamaya 7 will not give you much that the 6 isn't already providing.
//Björn