We build Java Spring MVC web apps for internal company users that have pretty basic GUIs. A few have jQuery/Ajax to add some GUI richness, but nothing major. For a new app, the business users are asking for a very rich web app (ex: drag and drop functionality, flexible tables like in Excel, etc). Our users can only use IE8 and we can't run Java applets in the browser. Also we don't have the time to learn an entirely new language (ex: Flex), so the learning curve can't be too steep for Java MVC developers. This app will only be used by company internal users (~ 100 concurrent users).
I have looked into some of the newer Javascript MVC frameworks (ex: Spine, Backbone, Ember). My main concern is that I am not sure how well the resulting complex Javascript will run under IE8. A secondary concern is that we don't have experience trying to maintain a lot of code in Javascript. It seems like a lot of Javascript code would become a maintenance nightmare (non-typesafe, harder to debug, not as many tools, etc).
I have also looked into GWT and ZK. These frameworks reduce the concern about maintaining a lot of Javascript code, but the IE8 performance concern still remains. If we built a very rich GUI in one of these frameworks, would the resulting auto-generated Javascript work well in IE8?
What framework would you recommend that would meet the high-level user requirements listed above and also meet the non-functional requirements (small learning curve, low maintenance burden, good future support)? Has anyone tried to do something similar to this? If so, what did you use and how did it turn out?