Depending on any number of factors - including which member of the Ion family it gets retrofit into, the closed bolt kit typically sees between 1,600 and 2,500 shots a tank. Overbore, mechanical, no QEV gets around 1,600. Underbore, QEV, devolumized to high hell dump chamber gets the higher numbers (typically around 2,200, some higher). If someone hands you a single number and says that is what you get per tank, it is most likely a cherry picked example that will work for a few people in identical situations.
So depending on how you look at it, yes - just a bolt kit. That doesn't mean it will operate anything remotely in the same fashion as a stock G3 though. Could also operate very similarly just with a lot more thought put into the dynamics of the system - both fluid and kinematic. Like I mentioned - I have things that are just well refined variations on familiar systems and some things that are rather out there. We'll be trying both kinds before deciding which route to go.
Historically, most spools have been poorly designed with little to no thought put into fluid dynamics, kinematic responses, and conservation of energy. My closed bolt Ions can break a case for most users, and Lurker's Eigenbolt for Shockers can do it. Both systems were designed with efficiency as the ultimate goal. Both happened to be designed by actual engineers who understand the operating principles behind them - something that can, surprisingly, not be said for many systems throughout paintball's history.
A gun manufacturer also has to look at things in a much broader spectrum than aftermarket manufacturers do - they typically need to create a much more balanced gun that could do with a lot of tweaking in almost all regards - but will work out of the box in almost all environments and not die catastrophically the instant someone who doesn't grasp the concept of fresh batteries and lube being important gets their hands on it. Aftermarket gets to chase those specific targets for the people that want a focus on something.
And the case+ is a goal, not an estimate
I have little doubt that I can get over a case with some of the designs - that does not mean that the concessions that would be made to get there would be found acceptable to the masses. It's a balance that needs to be struck. I don't doubt Techt would have no problems cutting most of their bolt weights down significantly if the severe loss of durability was something they were happy with, all part of the balancing act.
Yes, springs are involved in some of those designs, but not all.