Now factor in support costs. I can picture it now:
Customer: Uhhh your app sucks it's so slow.
Support: Hmm looks like you're using IE6. We've worked really hard to make our app function in every browser, however if you'd like the best possible experience then please upgrade.
Customer: What's a browser?
The best solution is to use chrome frame. The installation can be made seamless and if your app is worth using then your customers wont mind the extra 60s it takes to install.