First, over half of people do not use IE8, so not sure where that stat is coming from. Second, not sure I'd call FF or Chrome niche.

Not paying too much attention to IE (IE8 specifically) is a strategy worth doing. It's a pain in the ass to support IE8 and not worth the extra development time in the short term. Plus, not sure if this applies to this startup (I'd say it does), users like those in HN (the majority of whom use FF or Chrome) are the users you want initially to use and support your startup. If they like it, they become quite the dedicated user, which really helps to get your startup off the ground initially. So targeting FF and Chrome users is hardly a bad strategy for release 1. Adding IE8 in the mix is way more frustration than it's worth.