I agree that thermal conductance is probably not really relevant (although it might, at least in theory, help to increase expected lifetime of the LED if it is driven above spec).
From what I gather a hard epoxy makes everything completely rigid (and potentially brittle), while a firm (yet not hard) potting material allows a minimal amount of flex and absorbs shocks (but admittedly my knowledge about potting material is limited).
The reason why I suggested this stuff is that Zebralight uses it, and I completely trust them in this regard. Sometime back over on CPF somebody asked Zebralight what components they use in their lights and they answered with a list from which it became obvious that they know what they are doing and always choose the best possible components and materials (e.g. Corning Gorilla Glass, MOSFETs from Vishay etc.).
But it is, if we are being honest, not all that relevant which potting material is being used (after all it is a very small light with few components). Any potting is better than no potting.
I don't know which driver IC the E01 uses, but I think it is worth looking into the Prema PR4401 and PR4402. These are tiny LED driver ICs for single battery (0,7-1,9 V) input with an output of 6,5-40 mA (depending on configuration). They only need a single external component, an inductance (although for higher efficiency it is better to add a few components more).
After a quick look at the datasheet I would recommend the PR4402 together with an 10 µH inductance plus extra components (two capacitors and a Schottky diode). According to the datasheet this configuration should give a LED current of about 23 mA.