The expression "largefile support" is a bit misleading. What it simply does is that it uses "long long" instead of "long" in order to store and compute file offsets. But, since a long is wide enough on your system to hold a off_t (both are 64 bits), it wouldn't make a difference. Your Python is already able to address files larger than 4GB, since it uses lseek() in all cases.

(so-called "largefile support" would be enabled if you compiled a 32-bit Python)

By the way, you can run:

$ ./python -m test.regrtest -v test_largefile

to check that the "large file" tests (> 2 GB) pass.