Before the drop-dead date and within the operating temps of alkaline batteries, you really shouldn't notice any performance difference with alkaline batteries compared to lithiums -- except that the lithiums are lighter and have a higher power density.

That said, if you have some emergency only gear and the batteries have been in there 5 years, you might notice a big difference. The alkalines may be leaking while the lithiums will still be good to go.

Yeah, I switched my lights and GPS receivers (less the Garmin 60CSx) over to lithiums. What was Garmin thinking when they designed the 60CSx to be incompatible with lithiums? A reason to upgrade to the 62? That's one reason I carry the ForeTrex 301 as a back-up.