yeah - i got the muntons bock and wheat beer, and they recommend experimenting like :
add 1kg of sugar and a 500g bag of DME
add 500g sugar and a 500g bag of DME
add 2x500g bags of DME instead of sugar....
I don't think it'd have any effect on the alcohol, just the flavour. that said, 1kg of sugar and 500g DME should increase the amount of alcohol - more ferementables.