(Austrian here with a negligable collection of Classical music)
From a prescriptive standpoint, I don’t see a reason to prefer Sinfonie over Symphonie. Hell, even Symfonie would have something going for it. Guess they didn’t want to have the conservative concertgoers (even more) up-in-arms against them during the last orthographic reform, and left Symphonie alone (a number of other words lost their „ph“).
Regarding „op.“, is the general expanding of abbreviations out of the window, already? „Op.“ and „Opus“ (only one character more!) both are more logical to me, but „op.“ seems to be used much more often.
Finally, if we want to have this guideline applied mainly to Work titles, we should say so.