4AGZE... effing loud?

haha good luck mate, i have a full 3" system with the HKS headers on my aw11 SC and that thing is brutally loud. Good for waking the neibours up.

If you follow the path I went Mrskylighter you can easily fit 3inch. From behind at bumper hight all you can see is the tip, so you've got a good 3-4 inches of clearance before it gets ugly.
 
I'm surprised anyone would go for 3inch though, unless its turbo of course. Out of the dozens I looked at 2.5 was the biggest and the debate on forums was always between 2.25 and 2.5. To be honest I would rather 2.25 as I think 2.5 is too big but its a cost thing, 2.25 is expensive and hard to get parts off the shelf in Perth. 
 
Another muffler set up worth considering are the offset offset same end. They're chambered and baffled at one end and usually kill most noise and resonance, the only one I could find in Aus was from magnaflow and only in 2.5 inch at $330 ($510RRP). There would be others (Borla perhaps) as I know its used aftermarket for the Golf R32s and 996/997 911's; I've seen them on a few other cars too. It means no need for a resonator and a much shorter system but the muffler is slightly more restrictive. If it wasn't going to cost me 2/3s the cost of the whole system for one muffler I'd have done it this way. 
 
The last idea was a twin 2 inch with cross over into a cross over chambered 2 in 2 out muffler. Would have a tough sound.

haha, Yes, you will want a 3 inch then. My SW20 was quieter than this and it was 3inch dump into 2.5in back with a single 22in 10x4 muffler. The problem I found with the AW11 was fitting a cat and resonator, but you wont need the resonator with a turbo. There is plenty of room for 3in following the stock path though, with a big enough muffler it should be quiet until its on song. From my measurements before making this, you'd have room for a bullet (or stock) cat and 9x5 muffler. I struggled to get anything bigger than 16in that wasn't baffled but it's Perth, you have to special order everything here. 

If you want to wake up your 4A-GZE motor, the first thing you want to improve is the intake and exhaust air flow. In stock, factory OEM trim, the air filter and cold air intake are very restrictive and inefficent. The exhaust header is also restricitve and weighs a ton. Moreover, the same can be said for the factory cat and exhaust system's. To do it right, these all have to be replaced.
 
Way back when Moses wore short pants, and MK1 parts were readily available, I chose the following upgrade's:
 
HKS Cold Air Intake with a properly matched K&N Air Filter
 
TRD 4-1 Exhaust Manifold Header that I had ceramic coated before installation. This header can become VERY HOT
 
JDM spec Trust exhaust system
 
These 3 mod's greatly improve the air flow in the 4A-GZE motor. One drawback however, is the noise. Everywhere you drive, you get heard before you get seen.  Fortunately, where I live, I don't have to worry about decibal rating's or pollutant's in my  exhaust...... This kind of upgrade is not for the timid/meek SC owner, epsecially if you upgrade the engine even more like I have......
 
Bottom line is this. Not every SC owner can get legally get away with modifying his exhaust. However, IMHO,  if you can do it with impunity, and if you're OK with driving a LOUD car on the street, go for it!!!
 
It's great fun. 
 
Regards,
 
Minty AW11SC

Minty,
 
The OEM air filter is not as bad as you think. My turbo connects to the factory air intake and I am putting down 241whp at 17 PSI. Stock gze.