When i had my old 1.8 focus, i fitted a cosworth manifold, down pipe and sports cat with a milltek catback exhaust. Had an AmD map fitted on and got 150bhp with matching torque from a standard 115bhp. Cost me around £1000 for everything (cosworth stuff second hand off ebay).
If you have the money and want to be different, go for it, but if you are just wanting more power like turbo power, then it's really not that much worth it.
I only did what i did on the focus because not many people did it (now everybody does it, lol) and wanted to be different.