Correct use of "circa"

I understand the use of circa / c. as it applies to approximating dates. However, I have a writer who (over)uses the word in other contexts.

Examples:

... from circa early 1990's up until circa 8 years ago ...

... said population is circa 92-94% ...

... making one stop circa Pacific Coast Highway ...

... cost of circa $300,000 ...

... Rarely was I at either bar past circa 10:00PM ...

... circa 5 car widths from ...

Is this just my hatred of obfuscated language that's annoying me when I read this, or is it always appropriate to use "circa" when you mean "approximately?"

I wouldn't say it's always appropriate. Typically you only see it for dates (for example, "she was born circa 1920"). The Wiktionary article on 'circa' implies (but doesn't explicitly state) in the usage notes that it's used for dates and measures, so your examples of “circa Pacific Coast Highway” or “circa $300,000” appear to be incorrect.

And it definitely is pointless for your “circa 92–94%” example. Giving a range of numbers is already implying that you don't know the exact number, which means the “circa” is superfluous.

If you want my honest opinion, it’s one of those words used by people that want to sound smarter than they are. “Approximately” or “roughly” or “about” would serve just as well and not sound so stilted and stuffy.

Wiktionary states:

circa

approximately, about

Julius Caesar visited this area circa 50 BC

Usage notes:
Used only before a date or measure, never after

Note, two things, it's only used before a date or a measure. This means that if you were to use for location, it would be improper. E.g. Circa the Pacific Highway would be incorrect.

Because "circa" is used to mean approximately in dates and measures, people would naturally use it to mean approximately even outside of dates and measures, partly due to ignorance of the fact that "circa" is used only for dates and measures, and partly out of inconsideration of the rules of usage.

or is it always appropriate to use "circa" when you mean "approximately?"

No, it depends on what you are using "circa" to modify with.

It has always been my understanding that circa is properly used only when exact dates are unknown or disputed. (I will concede to my betters about the use of circa with measures. Presumably, the same rule about intentional vagueness applies.)

Using circa with an exact, verified set of dates is wrong. Recently, I edited a client’s work to correct “the poet John Keats lived c. 1795–1821”.