A few laps should be fine in the STI, keep at eye on the oil/water temps and it won't be an issue. The main problems I've seen on stock cars at Sandown are brakes not being cooled down properly.
2 x 60km->200km for each straight means full throttle for like 20 seconds each time.
Make sure you go easy on the gearbox and don't try to shave off time by smashing gears.
2 x 200km->80km (maybe 100 on the back straight, I don't know) also means the brakes get a big workout.
Make sure you don't superheat them, I had the adhesive on A1RM's fail, the pads completely disintegrated into powder.
Make sure you don't park the car with hot rotors, they will warp or crack if they cool unevenly, if you don't get a full cooldown lap make sure you do 3-4 laps of the carpark instead, and put a chock behind your wheel instead of pulling on the handbrake when you do park it up. Saw some GTR Brembos make a huge PING sound a few minutes after parking, and everyone that heard it just winced when they saw the source of the noise.
TLDR; Sandown, high speed braking, kills rotors, pads, tyres.
All easily avoidable if you drive to the (track and car) conditions