Full 3” exhaust vs half 3” with 2.76” on FI
 

So, my car is FI tuned up to E75 with 3” downpipe, 3” overpipe, 3” catted frontpipe and 2.76” catback exhaust, so would changing to 3” catback make a big difference?

Also, I know that air can only move as fast as the narrowest part of the exhaust, so is it safe to say that the 3” portions I have make no difference since my catback is 2.76”?

no...it depends on how often and long you are at load...
 

The real answer is "It Depends"....

At what point do you have a bottleneck? do you have an open wastegate or does it feed into the exhaust stream? How long are you at a high enough load to cause restrictive backpressure?

The fluids(in this case exhaust gases) will open up into the 3" portion and then at the 2.76" portion they will start to back up. The longer you have enough load to create enough back pressure through the whole pipe to limit power, the less power you'll make.

This is because the bottleneck will force the fluid in the 3" portion to start to back up but it has to get all the way back to the turbo/engine before the engine will recognize the restriction.

If you are no longer at enough load for the back pressure to increase all the way back to the engine(in a truly discernible way, technically the backpressure will START to increase immediately), and the gases can flow freely through the 2.76" section without unduly increasing back pressure, there should be no noticeable or even measurable difference.

Jaden

Tough to answer. Exhaust cools and contracts as it flows through the system, so a 2.76" rear section may not be as restrictive as 3" coming out of the turbo. On E75 the 3" pipe is probably restrictive too.

That is a common misconception. Exhaust gasses will actually flow faster through a narrower pipe, and have a lower local pressure at the same time. The problem is that smaller diameter pipes have greater losses to friction because of a higher surface area to cross-sectional area ratio.

Another thing to think about is the transition between the 3" and 2.7" sections. If this is abrupt, that's where most of your losses would come from. You are essentially stopping a portion of your exhaust flow, making it turn 90 degrees, pushing it into the rest of your flow, and turning it 90 degrees again before continuing.

So does 3" vs 2.7" make a difference, yes. More so in FI cases because more air means higher rate of loss for a given length. However, most folks in the real world don't see much of a difference from a catback on NA at least. Haven't looked much at FI exhaust comparison myself.