Looking through Jeepforum, found a post where someone saw a 2mpg increase.

On a question about the Neon injectors, do those have to come from the DOHC engine, or the SOHC engine?

 

Usually the only big gain someone sees in MPG from doing the upgrade comes from the fact they are swapping in newer cleaner injectors for ones that are often 15-20 years old. Swapping to the 4-port injectors comined with say also swapping to the 99+ intake manifold would be helpful MPG wise. If anyone wants to get some fresher injectors but not go to the "upgrade" ones I have a set of competely cleaned and tested stock 94 XJ injectors for $35 shipped.