What benchmarks have you run? Some benchmarks would show higher numbers but these are synthetic without having a real connection with actual performance. One example is the burst throughput when enabling write-back with Intel RAID controller; you get 2GB/s burstspeeds; but these aren't very relevant at all.
Does your OS contain anything important? If not, you don't need to backup and can use the RAM write-back feature without too much danger. If your array contains invaluable data, you should not use this feature.
Any crash or power failure has the potential to destroy alot of files on the filesystem with writeback enabled. This may also cause files that have not been modified for months to disappear. This is because the metadata on the disk will become corrupted; which can wipe whole directories. Recovering will mean you lose filenames, or directory structure.
I only recommend you do this on a RAID volume which could fail at any point without you having lost one byte of valuable data.