Python and R

I am just wondering if any of you are doing most of your scripting
with Python instead of R's programming language and then calling
the relevant R functions as needed?

No, but if I wanted to do such a thing, I'd look at Sage: 
http://sagemath.org/

It'll give you access to a lot more than just R.

Is there much of a performance hit either way? (as both are interpreted
languages)

Are you just asking, or do you have a particular execution time goal, 
which if exceeded would prevent doing this?  I ask because I suspect 
it's the former, and fast enough is fast enough.