rhel6.4 tuned profile virtual guest

We've been struggeling with some performance problems towards our SAN lately (high write latency), and it kind of matches well with the RHEL6.4 upgrade. I just noticed that 6.4 automatically enabled the virtual guest tuned profile, which amongst other things includes a quadrupling of the read_ahead_kb setting (from 128 to 512KB).

It's not clear to me when read_ahead_kb kicks in (anybody know?), but it looks like something that might have a major impact on a storage system when 150 VMs suddenly wants to read 4x as much.. What's the logic behind increasing this setting for virtual guests?

Has anybody else noticed any performance impact on their storage after upgrading to 6.4 guests?