Exhaust mods, computer chip, turbos with intercooler, nitrious injection, or you could have an engine and transmission custom built with all the bells and whistles if you have really deep pockets. Remember speed costs money.
Buying a WRX will be cheaper and faster than doing a lot of mods.
Other than add the 1.0 to get back to stock 2.5 for placebo effect nothing else is cost-effective.
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