The "added value" you might get if you update from Lood Delta to Look Keo systems is mostly lighter weight. The old Delta style pedals could be close to twice the weight of the newest Keos. As one noted already, stack height will be mostly noticed by racers who deal with much smaller tolerances than the typical recreational rider. I don't think float is an issue with the old ones anymore as I think the design of newer Delta type cleats provide lateral float even on the oldest Delta pedals. So if most of your pedals on your bikes are still Delta style Looks and they all still work properly, I don't think it would be worth trying to update to a newer or different system on some of them as you will just end up having to switch cleats all the time to ride your different bikes with different pedals, as the cleats are not interchangeable.
Chombi