Can I just "plug in and go" with a new ECU?
 

If I was to buy an ECU off here for my 51 plate WRX (coz frankly..it's too damn slow!! :D ) would I just be able to plug it in and drive away and thats it or do I need to have the thing mapped to the car? Also, if i was to just start off with a centre decat & backbox, how much more power increase should I expect to see?

To buy a second-hand ECU that has been mapped for another car, possibly with other mods done to it and running different fuels/booster and then bolt it on to your stock Imreza will not really work to your expectations!
If you want more from your car then the full decat and a good panel filter is a nice start, 'do' the suspension which will make the car a lot faster on a real journey, but if you want straight line go then (also) contact a reputable tuner for step by step advice to suit you, your budget and saleability of the car later on. Keep ALL the bits taken off the car!

Try Roger Clark Motorsport, TSL or Power Station and you wont go far wrong. I think you will be off in the wrong direction with your proposal.

Good luck,

911 :norty: