I don't have any listeroid-specific numbers yet (hopefully soon - all the pieces are in the garage). The theory is pretty simple: Higher water jacket temperatures offer three advantages:
1) they encourage boiling water and light distillates out of the oil
2) they reduce the running viscosity of the oil
3) they improve the completeness of combustion of fuel, and can therefore improve fuel efficiency.
cf: this paper as a typical example
http://www.ae-plus.com/Journals/Powertrain%20-%20Review%20of%20engine%20cooling%20technologies.pdf (pdf).
Possible disadvantages include: changing clearances of moving parts, possibly approaching structural limits of head and piston material, possible lubricant breakdown, and increased production of oxides of nitrogen (NOx)
For their part, Lister makes it clear in the operating manual that a hot CS is a happy CS. They were assuming nothing but water in the jacket and no pressurization, so "hot" would have to be assumed to be in the range of 180-205 F.
Given the low-stress design, I expect that you can go a lot above the boiling point of water with a listeroid before badness ensues. How much more efficiency you might get is open to question, but as long as the oil is thick enough to lubricate the rod bearing properly, I suspect that frictional losses would decrease, resulting in a small gain in efficiency. Of course, those of us with sleeved cylinders have to use a waterless coolant to go above the boiling point of water because a pressurized jacket is probably not a good idea.
One of the items on my list of things to do is to try running the evans waterless coolant above 200 F and see what happens. Before that, however, I plan on trying a thermal barrier coating on the comet chamber, piston top, valve heads, and combustion chamber face.
