Here are some tests of 7-Zip 15.14 (released in 2015-12-31) showing how it deals with relatives and absolutes paths in list files.
+=====================+=====================+==========================+===========================+
| content of list.txt | 7z a p.7z @list.txt | 7z a p.7z -spf @list.txt | 7z a p.7z -spf2 @list.txt |
+=====================+=====================+==========================+===========================+
| /tmp/dir1/file.txt | file.txt | /tmp/dir1/file.txt | tmp/dir1/file.txt |
+---------------------+---------------------+--------------------------+---------------------------+
| dir1/file.txt | dir1/file.txt | dir1/file.txt | dir1/file.txt |
+---------------------+---------------------+--------------------------+---------------------------+