We know that these factors affect hirelings:
1. The number of points in the hireling line.
2. The number of points in the materials line (that is, if your highest material is voidstone, your hireling will give you voidstone rather than iron)
Anything else?
Does your overall craft level affect hireling performance? If not, then a level 32 crafting skill for an alt would give you the maximum opportunity to get rare tempers, and there appears to be no point in getting beyond level 32 except for the raw mats (or potency runes).
What about other skills temper expertise, aspect extraction, etc.?