I was going to put this unter "Rules Questions", but I noticed that it's mostly just me ranting, so I'm posting it here instead…
Let's assume I have the improved version of the Wasp Familiar (either by being a 5th level caster with a regular familiar who took that feat, or by being a 7th level level character who took it twice).
The feat says to "use the statistics for an imp, but replace the invisibility spell-like ability with an unnatural lust (DC 14) spell-like ability usable three times per day."
The unnatural lust part is easy. But the "use the statistics for an imp" is… weird.
Let's compare: Here's the Imp
Is this wasp an outsider with the devil subtype? Because the imp is. (Magical Beast (Chaotic) could work, though.)
Also, I hope they don't mean alignment, because lawful evil doesn't suit Calistria's servant. (Same about the alignment subtypes, which should be Chaotic, if anything.)
It also has damage resistance "good or silver" - again marking it as lawful evil (chaotic creatures usually have their DR bypassed by Cold Iron, so "lawful or cold iron" would be appropriate).
And Calistria's servitor outsiders are usually immune against diseases, and resistant/immune to electricity. (Immunity to poison fits, though.)
Basically… the resulting stats would be a thematic mess.
2 quotes from the Wasp Familiar feat description-
"You gain a familiar as per the arcane bond class feature, using your character level as your wizard level. The familiar is a cat-sized, chaotic neutral wasp loyal to you."
"Special: This feat can be taken a second time by characters of 7th or higher level if they do not otherwise have access to familiars. Such characters have access to a wasp familiar that uses the statistics for an imp, as described above."
What the second quote means I think is that if you get the improved familiar version the Imp's statistics are used except "The familiar is a cat-sized, chaotic neutral wasp loyal to you."
So by the RAW the familiar would be a "thematic mess" as you say in all ways you point out except the familiar is Chaotic Neutral.
I have a first level character with Wasp Familiar. I discussed it with my GM and when I take the improved version we will change it to basically what you say it should be. But that is a house rule not the RAW.
Reminder: wasp familiar came from one of the splatbooks. Those splatbooks have a tighter word count restriction than say, a core book might. Expecting them to be high and tight as far as covering all the potential interactions and requirements is expecting a bit much.