Stats are just fine. As you can see there's a linear progression in terms of stats. Lvl 2 will be higher than other lvl 2 spy buildings, and so on. No increase is needed. You're getting more stats per bil spent on a vol, sure, but you NEED to upgrade further if you want to be as big as you can be. No two tiers of buildings are the same, nor should they be. Each tier has it's own functionality and stat weighting to cost. Comparing them with lower tiers doesn't make sense either - especially to a volary that's had a price cut.
I was told it was to "help close the gap" with the ridiculous price increase/lower plunder... Don't quite understand how this is gonna close the gap in any way, shape or form... But devs will be devs