its not a good idea because backup includes main logs folder whats contain error and access logs....for me contains 2 files aproximately 2GB size....
i have another idea....
@'/var/www/imscp/engine/backup/imscp-backup-all'
this: '--exclude=logs',
change to: '--exclude=$homeDir/logs',
ahhh....test: backup contains htdocs/logs directory but creates backup file with crc error:S 
This can happens if a file was changes while backup is running
Use varnish...for you its ideal:)
On my server aproximately have same daily users...but on multiple sites.
I dont use varnish now but for me is not sure varnish implementation getting to much "positives"
Look this loads:
http://i.imgur.com/fHSxQn4.png
http://i.imgur.com/iiZkvSH.png
Imscp in normal case was unable to work at this loads...
But after some configurations on php-fpm & apache works fine.
Why U use varnish? Because it "caching" your pictures in memory and the backup have more time to create the file before some file was "changed".
