I don’t care much for brand named silicone parts as they are likely all the same factory or just more money for the name.   You’ll need a trans cooler with the new radiator as the stock one has the cooler built into it. Just get one with built in barbs, don’t waste money on fittings etc. a forester one maybe two would be an option.   For the sump get a factory one modified. After that RCM has a rubber fitting to stop dents blocking the pickup. There is also a baffle plate that sits between the sump and block that just bolts in with the modified sump or stock one. https://rogerclarkmotorsport.co.uk/products/rcm-oil-pickup-anti-surge-adaptor https://rogerclarkmotorsport.co.uk/products/rcm-oil-sump-pan-baffle-plate for NZ sales try  https://jmms.co.nz ClubSub did a large group buy a few years ago.

Great advice from @Rossobianconero I've had a number of Subies and had them all tuned, you can so easily get caught up with a dyno number your chasing but at the end of the day whether it was 210kw or 225kw your still going to enjoy it probably just the same   As I've got older and probably still no wiser lol I like to focus a bit more on how the car handles like making sure the car has decent rubber etc. having power is great but no sense in having heaps if you cant use it and enjoy it properly 

Thats a good guideline, on our dyno (PBMS) GRB 6speed with the stock turbo would make anything from 220kw-235kw (295whp-315whp), auto is a different story, they make 200-220kw (268whp-295whp), there is sooooo many factors on what will actually make on the dyno, sticky tires, bigger heavier wheels, all of that will read lower on a roller dyno. The highest GRB that I have seen here with the stock turbo make 240-245kw, but one of the biggest differences, is that it was running 17" lightweight wheels, so the engine may be making the same as any other GRB but because of the wheels, it goes better, and the roller dyno show you that. I wouldn't be thinking about a number, I know that we live and die for the scale, the dyno, the 1/4 time, etc, but just focus on the experience, the fun on modding your car, and the driving pleasure of the results... best bang for buck is going to be, downpipe, turbo inlet, then maybe catback? or catback and CAI in the same place? to be honest at that point probably is going to be better with an aftermarket ECU. If you want to go to the track and keep it safe, probably a good oilsump is the best mod, and then just don't push lap after lap because the oil temperature is going to go through the roof and then... knock knock knock... rod bearing failure. Do not get me wrong, you can go to the track and have fun, but you just have to be conscience of a few things to keep it fun and reliable. As the tuner of PBMS, I would love to tune your car, but if anyone ask me, I will always recommend a local tuner (if there is a good one around), is just like your doctor, you don't want to have to travel 3-4 or whatever how many hours to see it, and less there is a good reason to choice that doctor to start with.    On the sti everything max out at the same time, turbo and fuel system, so if you are not going with a bigger aftermarket turbo, stock injectors and more than enough. Your car with a downpipe and a tune is going to be fun, and a massive difference to the stock car, I am sure you are going to be happy with that, if you go turbo inlet, choice the bigger you can find for your application.  

Brilliant   I'll go with   Invidia High Flow Downpipe Koyo Radiator ETS TMIC Silicone Intake Pipe AEM 340lph Fuel Pump AOS - IAG or similar

Exclude injectors, assuming you have 565cc ones they'll be fine for that power level. I would say definitely do the radiator, mine really struggled with heat on the dyno with the factory radiator, we put a cheap alloy one in there and it was fine until I went chasing power again. I would do a Koyorad if I did it all again, still plan to at some point.   I think a silicon intake is a worthwhile mod, the factory one is kinda awful. There's a few options, just get something branded not some bingbong Temu s*** as it will fall apart.