I know that look myself. But most experienced web coders know how to whip IE into submission, so I'm very skeptical about the 30-100% figure. (I usually guesstimate it at around 20% for a normal website.)
65% of their traffic is from Mac users, so this sounds like a good decision given their audience.
Nah ... that's about right, I've been doing html/css for almost 9 years and hard core javascript/ui stuff for another 4 or so.
Almost without fail, fixing stuff for IE takes as long as it does to just build the feature in the first place (so 4 hours to do a layout then another 3 or 4 to get it working properly in IE), with CSS/html layout that number has dropped as IE6 has been phased out, but I find that its actually moved over into doing js stuff in IE8 and 7.
The solution may be to design your site and test in Internet Explorer before even looking at it in any other browser. It might be faster for you to resolve the quirks in FireFox & Chrome then IE. Reversing your process might speed up your development time by 20%.
No, that introduces the same type of bias, just from the opposite direction. The key is to keep a broad range of browsers quickly on hand to regularly check _while_ you develop.
There's no reason not to have multiple browsers quickly available - modern computers have sufficient processing power to run multiple applications simultaneously these days.
Can't you surround your CSS includes with conditional comments to load ie-specific stylesheets? I'm not a web-designer by any stretch, but I thought that was the usual way to work around IE-specific quirks.
65% of their traffic is from Mac users, so this sounds like a good decision given their audience.