LifePO4 batteries for motorgliders - are we there yet?

Assuming this is a DG800 I'd leave the (4) engine start
batteries well alone; my experience is that they last about 10
years. The batteries, bought from DG, are hardly cheap but as
you are probably not in Europe you can probably fit a set for a
fraction of the money the DG agent wants. Cheap and simple
known technology.
If you are in Europe it's not legal to change these batteries to
Lithium types; secondly it's expensive, thirdly it will require a
new "weight and balance" as the c. of g. will change.
Better to change the DG wiring so that all your hungry avionics
runs from one (or two) "soaring" batteries (NiMH or Lithium
type) on the luggage shelf. You can charge these from solar
cells on the engine doors.
I've run this system without any problems. The important point
to remember is that DG have wired up the aircraft so that
everything, except the electric vario, runs off the (4) engine
start batteries! Personally I think it's a daft way to arrange
things: if you run the radio, transponder, horizon etc then your
critical engine start batteries are depleted (even if you have
the optional solar panels on the fuselage). Don't be taken in by
that cute battery switch labelling, whatever its position most
everything is still depleting your main batteries.