I'll start this thread off by listing the pressurization systems/pumping systems I can currently think of:
i) piston pressure
ii) air pressure
iii) CPS/bladder/diaphagm pressure
iv) spring pressure
v) compressed gas (well, it's technically the same idea as air pressure systems, just usually operating with a higher reserve pressure)
vi) peristaltic pump (in a weird way, it's similar to having a modified piston.. sorta... but it's not a piston)
vii) motorized (ok, this one can be a combination of the above)
viii) hybrid systems (combining attributes from a few things)
In terms of sheer absolute potential power, compressed air/gas systems have the highest possible. CPS/diaphagm and spring systems are limited by the strength of the rubber or spring used. One can achieve a lot more pressure using gas.
In terms of pumping, the problem becomes a balance of how many pumps, how hard are the pumps, and how much of a shot do you get after all the pumping.
In 2005, the Water Warriors line introduced the Pre-Charger system. The idea behind it was to increase the available upper air pressure limit while keeping the air and water apart. While the streams behaved similar to a CPS-based stream, a major problem was pumping. The pre-pressurized rear chamber made the blasters too hard to pump. With some adjustment, the Pre-Charge system has the potential to rival or even out-perform the CPS-system, but it needs to be tweaked so that the pumping isn't rock hard.
While one can make larger or thicker CPS-chambers, there is a thickness limit in that a strong chamber needs to have stronger elasticity (which usually means thicker), but thicker rubber chambers have a greater chance of uneven power distribution and tearing. This can be overcome to some degree by using layering (as people do when K-modding soakers), but there, too, there's the physical limit of just how much can be stretched onto the bladder.
One can remove the pumping problem by using machine-compressed air/CO2 canisters like those used in paintball guns. However, to be safe, the blaster would then need to be fitted with a proper pressure regulator and much more durable parts which would drive up the price of a soaker.
I'd babble more, but have work to do... I'm sure others can fill in more info as well.
