the span won’t hurt SEO. Paulie has a good idea, the only problem being that the image will be cut off (its wider than the ‘o’) – but that can be fixed by adding left and right padding (approximately 8px) with a negative margin of the same value. You could then try positioning with center 18px – the center should work for all browsers, but the 18px may vary.
The problem here is that you’re relying on the font rendering to be consistent. As you’ve discovered, it isn’t from browser to browser and platform to platform (even in the same browser). Another option might be to use an image for the logo with alt text of ‘storiva’ – this also should not hurt SEO and will be read just fine by screenreaders (and spiders)