Warranty is not an issue if you've got good friends.
If you get the things done,i would expect about 300bhp.
Meercat full snakehead down pipe back 3 inch system £550
Andy F Ecutek remap.....................................................£550
GruppeS or GT Spec Header and up pipe.......................£450
Labour to fit headers at Greersport ..................£100 - £150
Bosche fuel pump supply & fit at above.........................£150
total £1850
It's a bit more than you thought but you can do it stage by stage.
Like start with the exhaust and fuel pump.The car will be fine as i was driving like that for about 2 months but had to get the CEL reset at Subaru.
Then headers and finally a remap.
Your car will to great at the end tho