"since so far" is correct and usable in written English.
You can use it to mean "up to this point." For example, "I have spoken to three people about the project, and since so far none of them have agreed, I will keep looking for more potential participants."
Debevec has been upgrading the design ever since — so far, there have been ten models of the light stage.
Third, it lets Mac fans enjoy Instant Watching (since so far, watching on your computer requires Windows).
The indictment means that the four former military figures cannot set foot in Turkey, but Israeli officials said they could still travel to other countries in safety since, so far, Turkey has not issued international warrants for their arrest.
I didn't give it another thought until I heard I'd been nominated for an Oscar for Kama Sutra, which came as a complete surprise since, so far as I knew, it had never been distributed in the States.
I accept that resorting to quotas usually denotes a failure to make anything else work, but since so far nothing else has, perhaps it's time we got on with it.
It's natural for us to see ourselves as being at history's endpoint, since, so far, we are, but part of science fiction's job is to remind us that it's early yet, we're still a primitive people, the Golden Age may lie ahead.
Nothing his government has done since has so far made much difference.
It has been telling me what to do ever since, and so far it has never been wrong.
The sub-discipline has continued to flourish in the decade since, seeping so far into the mainstream that its disciples no longer constitute a self-contained school.
Mr. Porter printed his first flying car a year after the movie came out, in 2005, and the images have been so popular that he has made about two a year since — 14 so far, most in editions of five.
Mr. Bo, who was suspended last April from the Politburo and has not been heard from since, has so far remained in a parallel justice system reserved for the party elite.
