iOS 27 beta 5 has further expanded the new Siri voice customization options. In earlier betas, the new Pace and Expressivity sliders were only available for the two newly-introduced Siri voices that have an American accent. You can customize Pace and Expressivity by going to Settings -> Siri -> Voice and dragging the sliders. Apple previously indicated that the new Siri voices take advantage of a new powerful on-device model, but it has pretty recent hardware requirements. Siri AI itself requires an iPhone 15 Pro or newer; you just get the older voice synthesis on older hardware.