I've personally found the most use for her in rift battles, since she's a pretty forgiving character with her vampirism and meter control. However, timing out is still an issue for her even at around 15k. I will say that Diva can be a pain on defense. Usually not, because yknow, AI. But once she gets Sekhmet mode, it's hard to come back from that.
This is similar to my experience. I put off taking Diva to diamond for a long time, since I like her, but as awesome as her SA can be she just didn't seem likely to be able to hold up in high streak prize fights. And my experience with her as a diamond has pretty well confirmed that. She's not terrible, just... lacking compared to other options like Poltergust, Primed and Parasite Weave.
In Rift Battles she rocks though. Particularly that node where the enemy always has immunity, and there's limited options for not getting BB3'd to death if there's a strong diamond on it. And she has the ability to easily finish fights with full life, an easy 20 hit combo from her BB3 and the blockbuster finish bonus because her blockbusters are always full, so she rakes in a ton of points.
I guess she's okay on defense too? Either one of her MA's can make her a pain there, but particularly the health one if you max it out.