Honestly, the second cat after the turbo is the biggest restriction for WRX. Get either high flow cat or decat to see the biggest increase in HP.
I got 260hp with my bug where I only got decatted downpipe and remap. If you keep the stock catalyst, sure some adjustment in map can be made for up to 240, I wouldnt expect more than that with our petrol.
As mentioned above, stock intake is well capable up to 350hp and maybe even more. No need to spend any money there, and defo avoid oil based filters, as they fcuk with MAF sensor.
To get the most from mapping money(400e) keep buying stuff without installing it, and when you are ready, then install them all at once and drive to remap asap.
As Ciaran said, fuel pump is good enough up to 280hp and basically you will see the need getting bigger injectors before pump. Best of course, getting both.
From my previous experience, you have 2 options.
Highflow cat/decat
remap
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260hp
better turbo(either small with ball bearings or larger one)
injectors
fuel pump
3port boost silenoid(optional on previous setup)
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keeping same catalyst 270hp, decat/hiflow 300hp(depending on turbo size, even more, but then the gearbox will be limiting factor and you will need to baby it over 300hp)
Stage 1 exedy clutch is good up to 340hp, so you should be fine on that front as well for a while.
remember, that with changing to bigger turbo, characteristics of the car and engine will change. Bigger turbo takes longer to spool, and might not be as good for 'ass dyno' as small fast turbo, that gives proper push, even though its short.
Also, subaru engines are generally known for not liking high revs, so having huge turbo that will start spooling at 4000rpm and give you peak at 7000rpm isnt ideal on stock internals. With that in mind, I would try focusing on the sweet range of 3500-5500rpm to get the full boost