Sublayer (of MapImageLayer) currently only has a queryFeatures method. This is limited by the default number of records returned (typically 1000). It would be wonderful if Sublayer also had a queryObjectIds method like FeatureLayer, which is not limited by the default number of records set in ArcGIS Server.
e.g. if there were 93,000 features in a sublayer, you would get an array with 93,000 object ids.