As a rough rule of thumb under normal conditions at 10,000ft an A320 can climb about 2,200 feet per minute, at 30,000ft that goes down to about 1,000 Feet per minute.
At the 38,000ft that the aircraft is reported to have been at I would have thought 8,000fpm would be at the top end of a reasonable estimate.
There's not much accurate information in the press about the speed of the aircraft or how long it maintained that rate of climb for , or it's actions before that so I think we need to wait for more details before things will become clearer.