Does the feat Deepsight limit the creature's darkvision range to 120 ft. even if the creature has one or more ways to increase its darkvision beyond this range?
Deepsight
Your senses are especially keen in the utter darkness.
Prerequisites: Darkvision 60 feet.
Benefit: Your darkvision has a range of 120 feet.
Normal: Darkvision normally extends 60 or 90 feet.
There's no mention of the feat in the FAQ.
Things to consider
A fetchling nightblade's level 3 class feature nighteye grants him a +30 ft. increase to his darkvision range. Further, the character could wear gloom sight goggles, which increase his natural darkvision from 60 ft. to 90 ft. at the price of light sensitivity. This would be darkvision 120 ft. without the feat Deepsight, but would it be darkvision 180 ft. with the feat Deepsight?
Also, although I don't expect the character to meet the prerequisites of the feat Aspect of the Beast (night senses), he could eventually wear a cat's eye crown (18,000 gp; 1 lb.) for maybe another +60 ft. to the range of his darkvision.
Best Answer
tl;dr -- You always use the highest absolute value for an ability or skill, then modify by relative values.
There are two types of modifiers: relative and absolute. Absolute modifiers do not stack, while relative modifiers may stack, depending on the particular RAW for a specific modifier.
Let's take a look at the absolute modifiers in play:
Gloom Sight (or goggles): This feat and item each grant your character exactly 90' of darksight. No more, no less. This is an absolute limit. The goggles even state that they have no effect on Gloom Sight.
Deepsight: This feat grants your character exactly 120' of darksight. No more, no less. This is an absolute limit.
Now, let's look at the relative modifiers:
Nightblade 3rd level: The RAW says you gain darkvision 30', or if you already have it, an increase of 30'. This is a relative modifier. It modifies whatever your absolute value is.
So, you can only use one of Gloom Sight, Gloom Sight Goggles, or Deepsight. There's no benefit to using two or more of them together, because the RAW specifically states that they set an absolute value. If you later gain a more powerful version of darksight (Nightblade's 14th level, for example), then Deepsight no longer grants a benefit either. The most powerful absolute value is the only value that matters. All lesser values are dispensed with.
However, you can still benefit from the relative modifiers. This means that at 3rd level Nightblade, you'll have 150' of darkvision with Deepsight, and at 9th level, you'll have 180' of darkvision. At 14th level, this all becomes pointless, because you're no longer limited by darkness at all. You can see things from miles away if the atmosphere allows it; darkness no longer has an effect on you, so it may as well be bright as day all the time from your perspective.