#1: yes, you can back-up your data in some way to your SD and then restore it
#2: Don't know if you can search the archive (i suppose not, at least not with the Vodafone-Italy ROM) and sync all may and may not slow down. It depends on how many entries you got into calendar (you were talking about calendar, right?) and how fast is your connection. I use only wi-fi over my ADSL connection so sync takes really few seconds. From the point of view of Android overall speed, i have almost one hundred entries and i've noticed no sensible slow-down.
Hoping this helps.