I'm curious for a bit more details. Was this an outage where you could not connect (using their software, or via FIX or whatever API they provide) to close out your positions?
Do they admit the exact duration of the outage? Was it a complete outage - that is, customers (or at least you) were unable to connect? Or were you able to enter orders that were confirmed and went into black holes? I assume not the latter, in which case they would clearly be liable for the entire difference.
It's hard to say that they should be liable for the entire difference if you were totally unable to connect and had a position that lost value during that time window. Yet obviously they have a significant liability.
Was your loss due to bad luck combined with the outage, or was it a trading strategy that necessitated a quick out that was impossible due to the outage? The greater your ability to prove what orders you would have placed, the greater their liability now.