2000 Ford Taurus leaking transmission fluid and slipping gears?

Hi, so me and me friends did a muffler delete on a 2000 Ford Taurus SEL V6 with 76k miles. It sounded so sick that I kept flooring it on every street. Next thing I know I park it and it starts dripping a continuous stream of tranny fluid from the bottom. It slips gears really badly now. It doesn't make sense to blow a tranny in one session of driving hard so I dont think it has to do with wear on the transmission, what could the issue be? Cheapest way to fix it? When it's cold it isnt leaking or slipping just when it warms up it does

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