My friend has an STi he says by gettin a turbo back exhaust it will give him 23+ wheel HP i dont think tis is ture how would an exhaust system give you 23+ at the wheels. My cat back on my cavy gave me 10hp at the crank

That sounds about right! There are three cats on the new STi's. By removing them the exhaust gasses have less resistance to overcome so get out quicker. Also it allows the turbo to spool up quicker, so the boost comes in lower down the rev range.
dave