Just remember that there are no effective ways of making the IS200 more economical, so don't waste money on pointless nic-nacs like Induction Kits, Exhaust modifications, Filters etc all of which claim to improve mpg but do not and don't stand up to proper testing. The only ways to minimise fuel consumption are the usual - correct tyre pressures, everything running properly, brakes that do not bind etc., and keeping the weight of crap you lug around down to a minimum. Most importantly learning to drive it like a smooth six and not like a stolen Fiesta on a track day.
Enjoy it. The IS200 is a great car and set new standards.