I agree with go_the_datto and lumix6 this sounds like the alternator is functioning correctly.
An alternator works by keeping the battery voltage at roughly 14.2V.
If you turn on your lights this will use power and the alternator replaces it.
The more current that the accessories on your car use, the more current that the alternator will try to produce to replace it.
The more current that the alternator produces, the tighter the alternator becomes to turn and this is why your revs drop and also why your car stalls at low revs when you take off.

( Just means your engine doesn't have enough torque for this down low!
No big deal!)

One thing I like is what Niscoupe pointed out about a bigger pulley slowing the alternators revs down.
This could also help in your case 1200rallycar.
By the bigger pulley giving more leverage to the alternators rotor, it would require less torque from your engine to turn it.

Either way the problem you are experiencing
is no big deal. Just means a few more revs are required when taking off.