8.5 and 8.5.1 interoperability - can I design in 8.5.1 and run on 8.5?

I have not myself had my hands on the new 8.5.1 Notes client+Designer, but from what I hear, it's a great release.
At Convergens we are in the early stages of developing a new Composite Application-based user interface for our document management product. And I am wondering about how or if 8.5.1 should have influence on our development.
I am pretty sure that we will not be in a position to require our customers to run anything newer than 8.5 Fixpack 1 (not just yet). But of course we could use the 8.5.1 Designer client.
Doing so of course would require us to navigate around design elements and features that are introduced in 8.5.1. For instance if there is a new property to NotesSession, we would have to keep our hands off that property. And Xpages in the Notes client would be off limits.
My main reasons for even considering going down this path are these two:
- Improvements around Composite Applications. - The new LotusScript editor.

I am pretty sure that the Composite Application Editor in 8.5.1 has improved in several ways. We have - with both 8.5 and 8.5 FP1 - experienced some time consuming problems when editing a CA.
I would also guess that the CA editor has had some improvements that make it easier, quicker, slicker when you wire and work with your CA.

The new editor for LotusScript also sounds like what we need. We have many home made LotusScript classes, and working with these sounds like it will be a lot faster. Once we learn how to use it of course.

My experience with creating Composite Applications in 8.5 and then running them in the 8.0.2 client have been without issues....

Research is coming up.