Are there any gains to be had for cleaning the grime from inside of the i/c piping. What about the intake and exhaust manifold? How much does carbon build-up really restrict the car?

 

Carbon or varnish buildup around the throttle plate can cause it to stick - probably not worth cleaning until it happens. Gunk in the PCV va;ve can stop it from working and pressurize the crankcase. So it is probably worth checking as part of normal maintenance.