There could be a couple of reasons:
Are you sure about the quality of petrol you're feeding your ride with? Give her some hi-octane for some time. Carbon fouling can happen anytime.
You've been flooring the car as per your own statement. Well, at very high revs, max fuel is injected into the engine and some of it may may not burn completely resulting into black soot at the tail pipe.
Also check your air filter and plugs. I reckon you may have to go in for a tune up too.
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