The exhaust system is worth 8 points. Stainless Steel has an exception for the driven class. No lost points on driven, should lose points on trailered.
For trailered, the number of points lost depends on those judging the car at that point in time. Daring to transition to the realm of opinion, if an SS system replicates the original design (except for the material), I would probably deduct 4-5 points. If the system was an obvious aftermarket system, I would take off all 8. But again, there is room for interpretation at the show for those judging your car. So if I were making the decision for my car, I would be mentally prepared to lose all 8, and be happy if it were less.
If driven, I would deduct if the system was not factory-like. The number of points would be based on how aftermarket or custom it was. Glaringly different systems would lose all 8 from me. A factory-like system with an incorrect clamp here or there would lose a point or two. Improperly indexed (clocked) clamps would probably be noted with no points deduction.
Workmanship, cleanliness, and condition is another 8 points. For trailered, if SS I would take points off for the system as discussed in the previous paragraph, but would treat this category independent of material (not double dipping). If the SS system was "clean", even if an aftermarket Borla, no points lost here.
One man's opinion.