If I have the Diverse Obedience feat I can obtain the deity's boons of any list assumed I have the Deific Obedience feat and no levels in the evangelist, exalted, or sentinel prestige classes. But can I obtain lower level boons in one of such lists? For example as a Sarenrae worshipping PC with 16 HD I can obtain the "Fiery Spirit" boon, the "Healing Sunburst" boon or the "Channel Efficiency" boon. But let's suppose I'm not a paladin or a cleric and I already have fire resistance: can I obtain a lower level boon like "Healing Spirit" instead?
Technically, a lower-tier boon can't be taken instead of a higher-tier boon…
So far as I can tell from Inner Sea Gods, a lower-tier boon can't be picked instead of a higher-tier boon. Were such an option available, it would likely need to be mentioned somewhere in the feat Diverse Obedience as a typical creature only acquires a god's boons from one category (exalted, evangelist, or sentinel), and the creature does so in order. Neither the feat Diverse Obedience nor any other mention of boons in Inner Sea Gods seems to allow "dumbing down" a boon choice to a lower tier. (Or, for that matter, picking the same lower-tier boon more than once.)
I don't endorse the game design practice, but it seems that this is a case where if the character has the feat Diverse Obedience (and, by extension, the feat Deific Obedience) yet the player finds none of the boons that the god makes available at a specific tier acceptable, the player should've picked for the character a different god. That's not to minimize your frustration! (That's also not an attempt to turn you into a player who only makes choices based solely on their mechanical efficacy—Sarenrae might've been the perfect narrative god for the character even if she's not the perfect mechanical god for the character!) This kind of possibly-negative choice is endemic to Pathfinder and its antecedent Dungeons & Dragons 3.5. That is, some choices are better for some characters than for others, like which god a character should worship so that the character can receive the boons the player wants for the character. The game rewards planning ahead so that a character isn't accidentally or organically stuck with only bad or useless choices. Yes, that kind of sucks. (Also see this article.)
…But allowing a such a choice in the sample case isn't unbalanced
I can't imagine a situation wherein this GM would forbid a player from picking for his Sarenrae-worshiping character a lower-tier boon over a higher-tier one, especially as the character that's receiving that boon is level 14 or 16. Really, the ability to cast once a day even a 3rd-level spell—as many of Sarenrae's lower-tier boons do—is an ability that a player can forget his level 14 character even has.