Exhaust weight

Aluminum can be used after the headers but the corrosion issues can suck.

Titanium works great, costs a fortune.

Aluminum mufflers and dump before the axle will save you about a third. Titanium headers and head pipes will save you another third.

2:1 could save you some weight but you have to up size downstream which offsets some of the weight savings.

A good blend of cost and weight savings would be ti head pipes and aluminum mufflers and the shortest possible system.