Seems win-win to simply use chromeframe. Given this is a web app which presumably customers are already invested in using (since they're paying for it), requiring a 60s one-time install seems like the best course of action for everyone involved.

Slow speed of execution in IE might well be considered a vast improvement compared to the current state of affairs

Unfortunately the general public doesn't think like that. To the average Joe your site is slow therefore it sucks.