FYI the six speed box found in the IS200 is also used in Mk2 6spd MX5's, and Nissan S15 Silvia's incase you could get hold of them for less than £350.
If you have changed the fluid already I doubt changing it again will help, even to something like 140wt or similar.
If you just want the noise to go away and dont care about the current gearbox too much then you could do what some unscroupulus car dealers used to do back in the day to get rid of a car with a noisy box by filling you box with tights or saw dust :)
I am not reccomending the above really, although something like moly slip may be worth a try?