Any idea what the actual numbers to be gained may be on a 600hp set up?
There's really very minimal horsepower gain. I did some calculations previously and worked it out, might find it on a search. The biggest gain is going straight from a stock crank pulley with the IC fan on it to a lightweight crank pulley with no fan. All other gains are negligible.
As far as making the car look good, they look sweet! I have a full set of the RJC pulleys on the T.