Assuming Flash provides fairly clean access to the underlying hardware, performance should be as good as you can expect given the intermediate abstraction layer.
On a side note, it would be nice if Unity produced iOS apps that could be extended as easily using Obj-C / CocoaTouch the way it looks like you can work with native AS3 code.
On a side note, it would be nice if Unity produced iOS apps that could be extended as easily using Obj-C / CocoaTouch the way it looks like you can work with native AS3 code.