Been reading on here about doing this but a bit confused still.

I have a stage 1 T3 on my S2 and looking for a wee bit more poke and thought this might be a cheap mod.

If it is then great but what model saab does it need to come from?

Have spotted a couple of Saab 9000 turbos with code 452083-1S (or -15 maybe).

Is this worthwhile?

 

Not worth doing unless you change your comp wheel for a bigger one too, it's that which max's out first then the .36 housing, all you gain is lag.
This is why TT for exampe have stages 1-3 with stage 3 being the only one to have a .48 housing fitted along with a 55 trim comp wheel and 360 degree thrust bearing, stage 2 has a 50 trim comp wheel and 360 degree thrust bearing and stage one is just the 360 degree thrust bearing upgrade.