Hi all!
I picked up a Flash Flood the other day at the Toys R Us with the intent to use it for a casual water fight this summer. I bored out the primary output nozzle with good results, and was somewhat satisfied. But then I had this urge to do more, so I googled up supersoakers, and came across this site. Reading through this forum allowed me to get a very good grasp of what I was dealing with. I also remembered that I had my old Monster X from back in the day sitting around, broken (I'd opened it up and messed around with the trigger mechanism, ultimately rendering it inoperable). So I decided I was going to transplant the pressure chamber from the Monster X into the Flash Flood.
I thought I'd share a walkthrough with pictures to give back to the community and do my part.
Walkthrough
So I started with a basically stock FF, with the primary nozzle bored out, and the screen removed from the flood nozzle.
Opening the thing up was a fairly straightforward process: all it involved was removing all the screws. Unfortunately, there was one thing that had me stuck, and that was taking the lock ring off where the pump handle meets the gun. I had to break it off with a giant pair of pliers. It was a real PITA.
Here's a picture of the internals.

Similarly, I took apart the Monster X, and removed the pressure chamber module.

Next, I removed the pressure chamber from the FF. A comparison is shown below; Monster X top, FF bottom. Note the irregular shape of the FF PC. It has a bulbous shape in the middle. Anyone care to take a guess at how the bulge affects performance?

Looking at the pictures of the Monster X's PC, you can see that some sort of accomodation must be made for its length. The solution was to take a hacksaw to the back of the casing and have at 'er. I chose to cut along a pre-existing 'seam', which worked well.
The donor PC from the Monster X is then installed into the FF.

Finally, the casing is bolted back together.

Here it is, completely empty.

Here it is, fully charged. A full charge requires 34 complete pumps.

The new pressure chamber capacity is 550mL, vs the original 350mL.
Comments
1)With the clear PC module, the charge level is now clearly visible.
2)Range and power has increased substantially, but it's nothing drastic. The FF's pressure chamber's tubing was larger in diameter, but the containment casing for the chamber was also larger in diameter, so it didn't seem like there would be a great improvement to power, if at all.
3)Balance of the gun has surprisingly not been compromised too terribly much. The gun's design lends itself well to PC extension mods, because of how everything is laid out. Kudos to the FF design team.
4)The new 'rifle stock' does a lot for both aesthetics and ergonomics. The 'stock' also helps you brace against the recoil of flood shots, which can now be sustained for a reasonable period of time.
5)I'd like to see if there's a viable way to extend the reservoir next. The 1.1L tank is woefully inadequate, and only allows two full charges to be drawn from it, resulting in a maximum of three full discharges.