@f/2, it'd have an 67.5mm front element and could use 77mm filters easily. @f/1.8, the 75mm max aperture would mean 82mm (or bigger) filters. So personally, I'd rather have the f/2 version. Which exists. Could you explain why the 1.8 is so important? Btw, I totally agree on the IS. A 100mm@f/1.8-sized - non-L - 135mm (with or without IS) would be great too, btw. Nicely inconspicuous.