Was going to look at the 18-270 Tamron, but reports say it is a bit slow
It depends on what you mean by 'slow' - in the aperture sense, this rumored Canon will only be 1/3-stop faster than the Tamron (which goes to f/6.3 at the long end, and 'tricks' the Canon AF system into thinking it's an f/5.6 lens so AF still works). If you mean 'slow' in the autofocus speed sense, it depends on whether Canon puts USM into the lens. While I agree with Craig that they'd be foolish not to make it a USM lens, Canon has done many things that have been considered foolish (though likely there were market research-based reasons for Canon's decisions). If it's a standard micromotor AF, likely the Tamron lens with the 'PZD' AF (micro-USM) will focus faster.