Not sure about the legalities of removing the factory speedo, but the Blitz unit does display accurate speed. If you are after accurate and controllable boost with lots of additional functions you certainly won't be disappointed.

I used the Apexi AVC-R before the Blitz one and the Blitz leaves it for dead. It's dead easy to set up and you can bring the extra functions in as and when you feel comfortable.

It also has an over-boost function, where you set what boost you want to run and where you want the over-boost limit set. If it tries to exceed that limit (which it won't unless its VERY cold etc) then the screen flashes red, the unit beeps and it will immediately back the boost off to a pre-set level to safeguard the engine.