I am happy with the results from my 70-200 F4 L and 17-40 F4 L, but can't provide any quantitive measure of their quality sorry - I'm not that scientific about it. I would say there is perhaps a little violet fringing in both on the brightest stars? I can't compare them to APO, never taken a photo through an APO to compare. There's barely any violet compared to my Megerz refractor, at a guess 5-10% of what I get in the Megrez.
Personally I believe that when comparing L series lenses and those from other manufacturers in the same quality, the comments saying zooms are not as good as quality as fixed focal length lenses are "old days trash talk". That is - back in the old days zooms were worse, now in this L series type quality range I believe the quality does not differ and the convenience factor of zooms is obvious. I think there's a lot of reasons for this including technology advances and the great demand for zoom lenses compared to fixed focal lengths driving the quality of zoom lenses up to match fixed focal length alternatives. Fixed focal lengths often have the advantage of being faster - F1.8 through F4 where as the zooms tend to be F4 - F5.6.
Perhaps might be of interest to you - all the Red Bull Air Race shots on my web site were taken with my 70-200 F4 L used together with a cheapo Tamron 2x doubler - the results surprised me with their sharpness and colour. I suspect the cheap 2x would introduce false colour etc when used for astro work, but I'd hope the $800 canon one wouldn't.
http://www.rogergroom.com/rogergroom...y.jsp?Item=522
http://www.rogergroom.com/rogergroom...ges/000528.jpg
http://www.rogergroom.com/rogergroom...ges/000471.jpg
http://www.rogergroom.com/rogergroom...ges/000472.jpg
My 2c worth
Roger.