Going from a 5spd to 8spd in the Grand Cherokee's with Hemi was worth a little over a second 0-60 and near a second and a half in the 1/4. I believe it was even more significant with the V6 and it's narrower TQ band.
The biggest difference is simply driving around, or towing... when it's got the ability to shift for ~300rpm difference it just always feels "there" and never has to "scream" to do anything.
My biggest gripe with my Suburban is that it needs at least two more gears. Would be a night and day difference. A 10 speed? That would be badass.
