So I have 16 GB worth of XML files to process (about 700 files total), and I already have a functional PHP script to do that (With XMLReader) but it's taking forever. I was wondering if parsing in Python would be faster (Python being the only other language I'm proficient in, I'm sure something in C would be faster).
There's actually three differing performance problems here:

The time it takes to parse a file, which depends on the size of individual files.
The time it takes to handle the files and directories in the filesystem, if there's a lot of them.
Writing the data into your databases.

Where you should look for performance improvements depends on which one of these is the biggest bottleneck.
My guess is that the last one is the biggest problem because writes is almost always the slowest: writes can't be cached, they requires writing to disk and if the data is sorted it can take a considerable time to find the right spot to write it.
You presume that the bottleneck is the first alternative, the XML parsing. If that is the case, changing language is not the first thing to do. Instead you should see if there's some sort of SAX parser for your language. SAX parsing is much faster and memory effective than DOM parsing.
Is XML parsing in PHP as fast as Python or other alternatives?