The head tube remains the same and all it is is a larger diameter at the top of the steerer (1″1/4 top and 1″1/2 bottom). Of course this means a different steerer tube on forks is required. RockShox, Marzocchi and Fox have all worked with Giant on this to make forks available for the big production run of Giant bikes. Giant has its own batch of stems to fit, which they also say can be used for 1″1/8 forks by using a shim (what the?), but it will be no time until all your favourite small component manufacturers catch up.
30% measured using what specifications head tube length etc remains to be seen, but 30% is a massive increase so until I feel every percentile of that magic number then I am going to stay very cool to this development. Expect Spoke to be doing some extensive testing using some 800mm bars and different set-ups to gauge the difference.