The fuel mixture does not account for the performance. The fuel mixture is getting enriched when the engine is cold in order to account for the decreased atomization of the gas molecules. In other words, the mixture is the same as any other time and not any richer. To put it in another way, when an engine is cold fuel does not evaporate as efficiently and the engine is running lean. That is why the enrichment injector supplies an extra dosage of fuel during this period to compensate for the otherwise lean mixture and bring it back to normal until the engine is warm enough to do so on its own.
If you were to turn on the enrichment during normal (warm) engine operation, the performance of the engine would decrease since the mixture would not be optimal for efficient combustion.
Now, the reason your car has better acceleration has probaly something to do with your transmission. When the oil is cold the oil pressure inside the transmission is higher (and so is inside your engine as you would have noticed the oil pressure gauge shows much higher pressure in the morning when the engine is cold and until it warms up).
I suspect that this increased pressure inside your transmission makes it shift into the first gear. I believe but I am not sure (somebody verify that) that the 300E starts moving with the second gear. Probably the increased pressure in the tranny makes it start on the first gear, hence the increased acceleration and the ability to burn rubber.
As the oil warms up, the transmission starts shifting normally again and therefore you start moving on the second gear and bye bye burnouts...