The A58 is rated as 690 shots, the A99ii is 490, so if you are getting ~1000 from your A58, expect to get ~ 700 from an A99ii
The reason for lower battery life for mirrorless vs DSLR is simple - the EVF uses power, while the OVF does not.
The reason for lower battery life A99ii vs A58 is the A99ii is doing a lot more processing than an A58 for the larger image files, improved AF, etc. - but using the same battery (one thing Sony certainly has got right, keeping consistent with batteries, all the A mount use the same battery, the E mount has just 2 different batteries).