Though the Temple Guard do gain Stubborn, I believe it would be Stubborn LD 6 (the LD of the Stegadon model) though I could be wrong, as it is not often you join a character with Stubborn to a unit without Stubborn but higher LD.
I know that an Orc Warboss joined to a unit of Snotlings is still Stubborn LD 4, and that Korhil joined to a unit of High Elf Spearmen makes the unit Stubborn LD 9, but I haven't encountered this sort of situation before.
These two examples are inversly proportionate. The first refers to a character joining a unit with higher leadership than the character (IE skink character joining a sarus unit). The second is a character joining a unit with lower leadership.
Units always use the highest leadership present in the unit for tests. So a stegadon riding skink joining a unit of sarus recieves the benefit of their leadership (8). If they are stubborn, it is a stubborn 8. The skink has no effect on the unit's leadership. (BRB48)
However, stubborn units always use the leadership value on thier profile. The trick shows up in the rules when referring to characters. An orc warboss may use his leadership, OR the stubborn leadership for break tests (which ever is better). For panic tests and terror tests, since Stubborn doesn't come into play, they will use the character's leadership (BRB78).
A stubborn character joining a non-stubborn unit, makes the unit effectively stubborn if you choose to use the character's leadership for tests (recommended, if it is higher, especially for break tests). A non-stubborn character joining a stubborn unit will use the highest leadership possible (the character or the unit's) becoming stubborn or potentially not stubborn if the unstubborn character's leadership is better than the leadership of the stubborn unit's.
I may have worded that incorrectly, the stegadon's leadership isn't taken into account at all if it joins a unit of stubborn sarus. They are simply stubborn on their highest leadership value, and the stegadon reaps the benefits of it, along with the skink riding it.