Say you get at band 4 or 5 in extension 1 mathematics, would the scaling be just as good if you got a band 6 in just mathematics (without the extension)?
Extension subjects don't have band 6/5 etc, they have E4, E3, E2, E1.
Nevertheless, according to atarcalculator.com, 90 in 2 unit scales to 95.55, while 80% (40/50) in MX1, scales to 95.05. So yes, they're more or less the same.
A HSC mark (after alignment) of 42/50 scales to 40.9/50. Though keep in mind that all subjects scale down (bar ext 2 maths) and that the mean HSC score for Maths Ext 1 was 40.8/50 for HSC 2011.