Notice the usage, though - "In contexts such as re your letter, your remarks have been noted or he spoke to me re your complaint..." These are all in reply or in response to something past tense, and not used to initiate new subjects.
The etymology of the usage indicates that this has been is use as "with reference to" since 1707:
("with reference to," 1707, from L. in re "in the matter of," ablative case of res "matter, thing.")
It's a popularized technicality from Law, where it is used from the Latin to mean "in the matter of" or "in regard to". So in general the usage for which you are using at the beginning of a thread (or e-mail) subject title it is correct from a grammatical standpoint.
But, with that said, common usage is that when "RE:" is the beginning of a thread or email subject line it specifically relates to a reply, and using it otherwise will probably be confusing to users, as is the case here, many of whom may not be familiar with the term's history or denotation.
It would likely be less confusing if you were to use it somewhere in a subject line of a thread or email, but not at the beginning:
"My comments re: suitability of throwing someone under the bus"
instead of
"RE: Throwing people under the bus"
Incidentally, that's quite a core charge for those injectors. $625 for a 5-set replacement. A Silverstar transmission only has a $300 core.