so I finally decided to try this on the new L640XL. Took a long time to get everything setup but in the end and after a final hard reset and all updates I am very pleased with it. Aside from the time on the lockscreen and the startscreen showing slowly (a few seconds) I do not have any issues at this point. It defaulted to a large screen phone setting so I have all the screen estate (5 action item columns, 4 medium tiles on start page etc). I was surprised about that and thought I'd need to interop unlock to get that.
I've done interop unlock and was able to redirect maps to sdcard etc.
so far I have not noticed lags in notifications to the Band and will continue to observe. Other than the known limitations such as Cortana not working with Band, it seems to be my best experience so far (knock on wood), better than my experience with previous builds on my L830.
cheers
edit1: Noticed that I am getting some notifications twice, more so when I open an app with notifications on the phone.