I have 4 reasonably complex r scripts that are used to manipulate csv and xml files.  These were created by another department where they work exclusively in r.  
My understanding is that while r is very fast when dealing with data, it's not really optimised for file manipulation.  Can I expect to get significant speed increases by converting these scripts to python?  Or is this something of a waste of time?
R data manipulation has rules for it to be fast.  The basics are: 

vectorize
use data.frames as little as possible (for example, in the end)

Search for R time optimization and profiling and you will find many resources to help you.
R or Python for file manipulation