I get what everyone's saying, that 2.5 inches after the down-pipe is plenty, but I can't agree with the following:
This doesn't make sense. If the neck down is before the split, you're going down to a smaller pipe when there is only one pipe to flow through. This is a much smaller restriction than if you stay at 3" until after the split when there are two pipes to flow through.
Once you're flowing through two pipes, going down to 2.5" is less restrictive because you only need to flow half the exhaust through each pipe at that point. Maybe the difference is not significant at stage 2, but there certainly is a difference. I won't be staying at stage 2 forever, and if you've gone through the trouble of running 3" all the way from the down-pipe to the rear axle, why negate the benefit inches before the split?
I paid a premium for this, so I wouldn't be happy if they compromised anything to use up what they had lying around.
I will bring this up respectfully, but this isn't the only issue I had with this shop. I had other work done and found loose nuts and bolts, half seated hoses, hoses with no clamps, low coolant, etc. I even found damaged breather hoses reused and two washers on the oil plug, both causing oil leaks.