Yeah, there's nothing special about the nacelles on Excelsior. ILM designed several new Starfleet ships for ST III, and they simply varied the look of the components for the sake of variety itself. Grissom has nacelles that don't look like Enterprise's, Excelsior has nacelles that don't look like Enterprise...it all means nothing. And no one at Bad Robot is paying attention to the old visual continuity to the degree necessary for people to read grainy silhouettes of ships for "clues."
Fans make up explanations and continuity for these things, and then seem to expect the filmmakers to adhere to it. Nonsense.