With the solinoid disconnected your ECU wont be able to control when the wastegate opens. That means the you are relying on the spring inside the waste-gate to regulate the pressure. It limits your maximum boost to the strength of the spring and gives you no control over boost pressure.
For my money I wouldn't just trust the wastegate spring, You can get a solid BCS for around a hundred bucks, if it keeps the engine happy thats worth the money for me.
13 WRX Hatch - Cobb OTS Stage 1