Anybody of the fast movers want to speculate if the water injection really would get the Rhino up to Mach 3?
I could write a white paper on the reasons why not. Short(ish) answer:
Not with the current engines. Even if water injection enabled them to develop the necessary power without grenading (unlikely), the aircraft structure and inlet design is all wrong for that speed. Above Mach 2 or so, you'd have shock wave impingement issues, leading to loss of power and probable structural damage to the engine. The aerodynamic heating and loads would cause their share of structural damage too. New engines, ablative materials, structural upgrades, fuel system, ECS, and possibly FCS changes would be needed to even attempt a Mach 3 run. It would honestly be cheaper to build something from the ground up if your goal is a Mach 3 fighter. Or steal a MiG-25 or better yet, a YF-12. The Rhino is designed for maneuverability, taking iron downrange, and landing on the Boat. She can sprint some (depending on loadout) but just isn't built for the incredible demands of flight above Mach 2. I wish it were that easy- we could all be cruising around in sub-orbital flight on a regular basis.
tl;dr version: Water injection yields a small boost in power for transient needs (e.g. takeoff or hovering in the harrier). It's not going to give you twice the speed without a slew of other extreme modifications. It's a fun thought, though.