I'm going to go out on a limb and say that something else is wrong with what you're doing/trying to do. I import ~1400 records, comprising over 40,000 lines of xml, and I only split the XML file into 3 files.
If you up your PHP memory limit (512MB is well more than enough; mine is 128MB) and it's not importing your records properly, then there's possibly an issue with the way you have your fields mapped in your import configuration.
Alternately, you may have set your PHP memory allocation to 512MB, but it might not be set in the correct location or it might be being overridden by a config elsewhere. Create a php file that you can visit in your browser and add the following:
$memory_limit = ini_get('memory_limit');
It will output what the server is seeing as the PHP memory limit.
Finally, there while you can set PHP to have 512MB of RAM as its limit, this means nothing if you don't have that much available on your host/server. You might be running out of system memory, and not necessarily PHP-allocated-memory. Does that make sense?