Firstly, I have scoured the forums already for a proper answer to this question, and I haven't found it yet, so I wanted to place it here. The usual questions tend towards "Do it get a bonus to give to its master" and "Does a Witch lose its spells", and "Does a Celestial fox give its bonus to its master!"
And. I'm satisfied on that front.
But, I'm having trouble discerning on whether or not an Improved Familiar would be granted the bonuses of a traditional familiar, such as Shared Spells, Deliver Touch spells, improved natural armor, raised intelligence, and using its masters base scores when better than the familiars. (i.e. Base attack bonus, saves, skill ranks, etc...), half their hit points.
Or whether they just use the base monsters abilities. Period.
Improved familiars otherwise use the rules for regular familiars, with two exceptions: if the creature’s type is something other than animal, its type does not change; and improved familiars do not gain the ability to speak with other creatures of their kind (although many of them already have the ability to communicate).
So, type does not change and they do not gain the ability to speak with other creatures of their kind. All other rules are in effect.
Note: this creates an odd situation where the intelligence score can drop from the base creature's intelligence to the score in the familiar chart on CRB p83. Most GM's houserule this to be the better of the two.