If invisibility was useless Shilliew getting that new fruit wouldn't be a deal at all. There's only a select few characters where invisibility wouldn't bother them. It's an annoyance for majority of the cast.
Sanji who is already extremely fast in combat now has the added edge of being invisible. CoO seems to be a conscious effort for most characters, It's going to be harder to land attacks on him, and harder to avoid incoming attacks if he phases in and out of invisibility. Besides haki is finite, constantly having to overuse CoO just to see Sanji works in his favour anyway.
Aside from that, one of his biggest issues was his durability. Now he can comfortably block attacks whereas before even blocking an attack could change the tides of a battle for him (see Vergo vs Sanji, where Vergo actually didn't land a single hit on Sanji, Sanji outpaced him the entire skirmish yet Sanji was the one leaving with lasting damage). Or Sanji vs Kuma if you want a preskip example, where he was in pain after landing an attack on Kuma, both instances he didn't even take a clean attack yet was the one grimacing. With this issue solved Sanji can afford to block or even take attacks. King in zoan form overpowered him with a stabbing attack, yet Sanji came out unscathed, you add haki into the mix and Sanji does significantly better against weapon users now. His durability is now #1 in the SH crew, no one is taking that attack the way Sanji did.
Though I do think flight is redundant since he already had sky walk, but it still gives him a bit of a boost in speed, and its less taxing, could never go wrong with that. RS Sanji should be a problem to fight for virtually anyone in this raid, idc who it is, let's see how Oda handles it.