Later Ford 302 injectors flow the same rate as the OEM 4.0L (19#/hour,) but they have four smaller nozzles, which helps with fuel atomisation. So, I do believe there is a small increase in fuel efficiency and power output (but you'd have to do before/after on dyno runs to find out exactly how much. I don't think it's enough to replace them outright, unless they want replacing anyhow.)
Installing a "bigger" (flows more fuel) injector on a stock engine won't be much help, simply because the ECU will compensate for the "rich" condition by narrowing the pulse width - or injector "open" time.
Installing a bigger fuel injector is like installing a bigger carburettor - if you've bored/stroked the engine to increase displacement, it can be a good idea (size them to match the displacement of the engine.) However, simply installing larger injectors won't be enough help to warrant the work and expense (unless you're having it done for no cost in parts and materials.)
Save replacing them for when they're finally due - like the body seam is leaking, or something like that.