If not, drop me a line at jani at google dot com with a reference to your support case and I'll be happy to take a second look. (Yes, I work in Google Cloud Support.)
That said, we have very recently (as in, late October [1]) introduced a new pricing model for GCS with the explicit goal of reducing latency, and the SLA may be due for an update accordingly. I'll look into this.
[1] https://cloudplatform.googleblog.com/2016/10/introducing-Col...
Also, the HTTP 500 thing is specific to GCS only, other services like GCE [2] define downtime more broadly as "loss of external connectivity or persistent disk access".