The swirl does just that, swirl the intake charge to promote better atomisation, and therefore better economy/emissions, I would suspect at the loss of a bit of top end power. Also, the 1.2 has a bigger diameter head as it has a bigger bore! So, gains lost in putting a theoretically better head on a 1.0 are lost by the drop in compression.
FYI the March ST head is like the 1.2 head but as the 1.0 size but lower compression...
