Im considering adding a booster tankless hotwater heater, but the only one advirtised to act as a booster, or supplemental is the seisco electric. Which looks realy bad when reading cust. reveiws. Any positive experiences with specific brands , or is it not even worth it , Curruntly the upper element is the booster and I realize how that will effect the staging , when a second tank is out of the question ( space ) whats best?

I think any tankless should work well when fed from a preheat tank heated by a desuperheater - not sure if that is what you are getting at here.

The only experience I have is with the Bosche LP tankless , but according to the rep from our geo manufacturer , this wont work because then sensor that turns them on requires inlet water around 80 F or lower to turn on .As is the case with most whole house units , They recomend Point of Service units applied to the whole house , the only problem I have with that is they all look like POS ( not point of service ).

Any way my point is , Am I better off using the upper element to boost the temp ( even though its not right ) or are the savings significant enough to take a risk on one of these units

When I said "any tankless" I was drawing on my experience with electric, not gas. I can't comment on gas units.

The element temp differential method offers limited potential for successful application of a desuperheater to a single electric storage water heater. I've commented at length on this option elsewhere here so won't repeat myself now. I doubt the economics of that option would support a reasonable payback period of the increased cost of a system with a desuper.

Thank you