Probably been asked a million times but every forum I post on Facebook asking question I get a bunch of non serious answers. 

I just bought my first subi it’s an 03 Impreza wrx. What mods or upgrades should I do? I’m not turning it into a 500hp car but I’d like to add a few simple bolt ons. Thanks in advance!!

decat, panel filter and map is a good option for power

Suspension, roll bars, drop links for handling

 

just depends how much you want to spend :)

I agree with tidgy 

all dependant on what you wanna spend 

Or as below 

this is the major components required to get car to around 500bhp at the cost of £12-£15k

1) Engine removal and strip down
2) Closed deck conversion to block and drilled and tapped for the 14mm studs
3) Cylinder heads surfaced and drilled for the 14mm head studs
4) Manley con rods and ARP fasteners 
5) ACL bearing sets
6) Omega RCM pistons
7) RCM Head gaskets to suit the 14mm studs
8) RCM exhaust manifolds and up pipe
9) Precission billet turbo unit
10) Samco turbo inlet pipe
11) 12mm oil pump
12) 1000cc injectors
13) Adjustable fuel pressure regulator
14) Parallel fuel lines
15) Tumbler delete kit and welding
16) Vacuum pump removal
17) RCM cam belt kit
18) Inlet and exhaust man gaskets
19) Full engine gasket kit
20) 19 row oil cooler
21) Front mounted intercooler kit
22)ACT clutch and light weight flywheel
23) Turbosmart BOV
24) Full de cat exhaust system turbo back
25) AEM cold air intake system
26) NGK 7 plugs minimum 

 

does not inc brake upgrade which at that power is a definite!
 

The usual route for a wrx to get the most hp, before expensive drive train and engine work upgrades would be. ...

Panel filter 

Pink 565cc injectors  (from a newage sti)

Bigger turbo (from a newage sti)

Up pipe without a pre cat (aftermarket or sti)

Sti tmic.

Sports or de cat .

3 port boost solinoid. 

All those "mapped in" should see 330hp ish ,which is about the limit for the wrx standard non forged engine and 5 speed drive train . 

Try to go much more than that and you'll need to be saving for a forged engine  (£4k+) and a 6 speed box (£2k just for the box) 😉