1. What is the most energy efficient string material I can use for these limbs regardless of its durability?
A string made with any of the low stretch materials will be efficient. How it's made, how many strands, and what your bow 'likes' will make more of a difference. 8125 and 450+ have the least stretch/creep of any material I've used. Of the two, I much more prefer 8125.
2. Who makes strings from this material?
I know I do, but I'm sure several others do also.
3. Adding the proviso of durability to energy efficiency, does this translate to a different string material? If it does, please identify it.
Gaining efficiency generally means going lighter--i.e., fewer strands. I have gotten way too many reports of low strand count 450+ strings not holding up for me to be comfortable recommending them. Low strand count 8125 and Dynaflight '97 strings seem to hold up well.
4. Who makes strings from this material?
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Am I correct that regardless of the information I receive from the previous questions, endless loop bow strings are more energy efficient that flemish?
Not really. Again, it depends on how they are made, strand count, material, etc. I've made a lot of both, and on some days I rather make an endless than a flemish, but in the end the difference is negligible. The biggest difference I've seen on my bows is noise, and flemish has been quieter for me.