The Ravenscoft 275 is a piano app for iOS that I just learned about. It sounds truly fantastic. I do not do any of my audio power lifting on an iPad so I am not sure the price ($35.99) is worth it.
Apps like this make me dream for a day when the iPad can run a real digital audio workstation with full plugin support. I have dumped a lot of money into Waves and Native Instruments plugins over the years and I like having them at my disposal. iOS technically does support something like audio plugins. It is called inter-app audio apps (a mouthful,I know) and it allows third party audio plugins to be run in GarageBand. The app Audiobus allows you to make third party plugin apps work with third party DAWs (and more). I did a brief video demonstrating the feature a few years back. You can watch it here.