Fuel mileage is highly dependent on driving style.

Expect fuel mileage to decrease with winterized fuel.

There are specific symptoms that indicate that injectors need attention. It's in the archives. Wholesale replacement usually doesn't pay off. It's best to check the system thoroughly - chain stretch, IP timing, delivery valve and seal condition, filter condition, cleaning with Diesel Purge, etc. Even on replacement, I think the balance tips in favor of having a qualified shop clean, calibrate and balance your injectors vs installing rebuilt injectors. The quality of the rebuilt injecors readily available has been questioned.

Sixto
95 S420
87 300SDL