xPages versus Forms and ExtJS

So I’m really pumped about using xPages and upgrading from 6.5.5, but my company’s primary Domino web UI (as we only have a few) is leveraged heavily on extJS and works pretty well. Unfortunately it a read only web UI and the write portion is still in the client. As part of the upgrade we need to move off the current client portion of the UI to a fully functional read/write web app. Eventually, the Lotus Notes client portion of the app will be retired and only the web UI will remain.

I have read all the hype about xPages but the learning curve seems to be quite high. Other than the cool factor and using new technolgy that embraces open standards, will my app run any faster or more efficiently? Also can I easily leverage my current extJS framework, grids, ect. with xPages or is it lost?

Lasty, because my company is very risk adverse, I will be required to keep the client side of the application in its current form until the new web app proves itself. Other than obvoius development concerns, will I be able to use a form based client UI and a web based xPage UI working at the same time?

As you can see, I have little experience with xPages, but before I can commit to a sizable development effort and completely shift to our development to xPages, I need to ensure the risk is clearly worth it.