The MS3 is a very good autocross car, you will have a awesome time and really get to learn your car.
Remember to look a head, you should be looking 3 or 4 cones in front of you.
No sudden jerks of the wheel, try and keep it nice and smooth.
Keep your hands at 10 and 2, do not cross over your arms when turning, shuffle the wheel.
a new motor mount or the inserts would be a really good idea especially if your going to autocross, Cant go wrong with the upgraded SRI either.
Also do not be afraid to ask questions and even let someone else drive your car, When people run a better time then I can I know my car has more potential in the form that it is.
Bump up the front tire pressure, drop the rear a bit. Take out anything you can do without (spare tire, rear seat, etc.). Make sure you have less than a half or quarter tank of gas. If possible, find a vacant parking lot somewhere you can practice. You might have to fiddle with the tire pressures a bit.
+1 It seems like really obvious advice "look ahead"; but it's surprising how people won't look where they want to go, since their eyes are all over the place, like staring at the other car's bumper, or worse, staring at exactly where they don't want to go/crash into.
I can't really say anything about setup tips, but knowing the track and working on being consistent (getting the line and brakingpoints/apex/exits just right, lap after lap) is a good step, and dare I say at amateur level it will get you a better lap time than working on a stock car's setup. By being consistent you'll be able to get a feel of what's right for you and what's wrong, and from there you could go on and adress what you dont like, one change at a time. I'd start with working at the manufacturer recommended tire pressures, and start tweaking after you know what you want the car to do.
Work on a proper line, and once you get the car behaving just like you want to, speed through the course will pretty much be a consequence of your good work.
I ran about 42 front and dropped my rear to 30, the car has very good rotation, I have not been able to spin it yet, but have had some very nice movement. I really thing the car requires larger front tires, but with proper suspension you may be able to get away with something close to stock size. However I run against evos running 245's all around, and ohlins (one of them is prosolo ranked like 3rd in STU)
MY dad taught me an old autocross tip where you chalk a few lines on the sidewalls of the tires, then go and do a lap. When you get back if the chalk is rubbed off then your tires our rolling over and they need more air.
When I setup for an event I leave the spare tire and jack in. I run with about a quarter tank or less. Two to three pounds more air in the front than the rear. High pressure though, typically low 40lbs. I keep the back seats locked in the up position. And do the TC/DSC mod where you hold down the button for a few seconds while turning on the car. Thats a must.
Thats about it for the car. I've tried many different configurations this year and that was my best handling setup.
Remember this car loves understeer. So trailbrake enough to going into the turns, keep it in the high three with the rpm's, then let it rip on the way out.
as said above, smooth is fast. I work doing kart racing for a living and you wouldnt believe the difference it makes just being smooth. Smooth inputs for gas/brake/steering, and a good heal/toe will help. and always set yourself up for whats coming next on the track dont get caught up in each turn your at