Just for info, I borrowed a calibrated ammeter from work and connected it inline with the Alternator o/p. We were pulling over 50A at Idle with just the fans, water pump, fuel pump, ign and data logger on. I didn't try it with the lights on as well since the the shunt for the ammeter was only rated for 50A With the engine off and just the lights the current was around 20A, this time connected to the battery feed (it didn't read anything connected to the alternator ).
My conclusion was: If I had a race car I'd still run an alternator, though I do know folk who just keep the battery topped up between rounds and don't seem to have any issues