Too many sandy vaginas around here.
Make sure the intake manifold is re-installed with gaskets and that nothing else has been left off or disconnected (i.e. small things like the boost pills missing from your lines before can have a big effect later on).
I would recommend getting a service manual for your car and ensuring it is reinstalled correctly and at the correct torque settings. There are plenty of places on the net to grab a copy too i.e.
http://ken-gilbert.com/impreza-manuals
I think good on you for giving it a go, but make sure you have a good read and research prior to doing your own work. Usually you can find out the best way from others mistakes or by following the workshop manuals.