300 watts 230V  is ~ nearly 1.5 amps per lamp. * 6 is 9A. call it more like 10A at 208V.


Now compare the price, the size, and the weight,  of a 208V transformer with a 10A 230V secondary,  versus a  230V to 24 V 10A secondary, and you will see why an auto-transformer arrangement is vastly preferable when the amount to be added or taken off is a small percentage (in this case only about 10%) of the total.  The only thing I'd add, if by 2 phase you mean neither side is neutral, then the switching for changing the lamps needs to be double pole. And you may need  a bit of inrush limiting inrush limiter - an examples  to make the lamps last longer and reduce the risk of blowing out the supply on switch on - halogens are about 1/10 of the resistance when cold they are when hot - so your  300 watt lamp,  around 190-200 ohms or so when hot is more like 18-20 ohms for the first 10th of a second, and 6 of them will be quite a thud, as your 10A on load is 100A inrush. Do realize though that those simple limiters work by warming up from some modest no. of ohms to a much lower resistance state when hot, so need to be in a ceramic choc block, and keep the body away from  PVC wire.

300 watts 230V  is ~ nearly 1.5 amps per lamp. * 6 is 9A. call it more like 10A at 208V.


Now compare the price, the size, and the weight,  of a 208V transformer with a 10A 230V secondary,  versus a  230V to 24 V 10A secondary, and you will see why an auto-transformer arrangement is vastly preferable when the amount to be added or taken off is a small percentage (in this case only about 10%) of the total.  The only thing I'd add, if by 2 phase you mean neither side is neutral, then the switching for changing the lamps needs to be double pole. And you may need  a bit of inrush limiting inrush limiter - an examples  to make the lamps last longer and reduce the risk of blowing out the supply on switch on - halogens are about 1/10 of the resistance when cold they are when hot - so your  300 watt lamp,  around 190-200 ohms or so when hot is more like 18-20 ohms for the first 10th of a second, and 6 of them will be quite a thud, as your 10A on load is 100A inrush. Do realize though that those simple limiters work by warming up from some modest no. of ohms to a much lower resistance state when hot, so need to be in a ceramic choc block, and keep the body away from  PVC wire.