Well there is no specific defect for having a straight through exhaust system.. You can however be defected for being too loud, having no cat converter, or having the exhaust hang too low.
You'll get a red defect for having no cat converter (along with a VERY hefty fine).
You'll get a yellow defect for being too low (lower than 100mm at any point).
You'll get a yellow defect for being too loud (louder than 91db).
I dont know what the fines are for the above, except the cat converter which is about $10,000.
There's no defect or fine for just having a straight through style exhaust system unless it breaks one of the above rules... I'm sure theres other rules too but they're the ones I know about..