You have two main options really.

1. Upgrading pads and rotors as per suggestions in this thread. You'll be looking at around $1k to do that. Should net you pretty decent braking performance.

2. Hold on to your money and scour the second hand market for a good deal on brembos. On a lucky day you could probably get calipers + 4000 rotors for $1k ish.

Everybody I spoke to with experience in racing our cars told me to ditch the stock calipers. Too small, don't dissipate enough heat, and no matter the pad/fluid/rotor combo they always faded away.

I've done a little bit of track work on my modest T2 rotors and ns400 pads and it was decent enough to get out and have a go with at least. Perhaps a setup like that to tide you over until you can get something better. Cost me something like $300 to do.

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MY08 WRX 325hp/456nm/18psi E85