I find the same result in BE 10. I get 0.9:1 compression with 357-359 MB/min backup. I believe the problem is not the compression of files method but how they calculate it.
My Exabyte VXA-2 is capable of 360 MB/min. I have over 600,000 files in 500+ directories. They are basing the compression ratio on 360 / (357-359) and rounding to a single digit.
So 0.9 : 1.
Do you have your BE configured properly? Do you have enough RAM and CPU power to handle the backup? Is your tape backup on its own SCSI bus separate from your hard drives or SCSI RAID arrays? If you are only getting 0.5 : 1, I would look at your tape device settings. If your device is 64K packet based, try 64K, 64K, 32, 0. for your device setting. I also recommend defragging and error checking on the drive to improve backup performance.