Yes, methinks there are some remnants from the Pavoni article that seeped into that for the Achille (re: boiler pressure and drawing heated water into the cylinder). Thanks for pointing them out [Note to Colin to correct.]
As you state, in the Achille, the boiler is a closed environment (until you open the steam valve). Cool water is gravity fed from the reservoir on top to fill the cylinder. Lifting the lever transfers the water below the piston. Pushing down on the lever pushes the water from the bottom of the cylinder through the heat exchanger.
The heat exchanger itself consists of about five turns of pipe inside the boiler, of which only one to two turns (depending on the water level -- one at minimum, possibly two at max fill) are immersed in the boiler water and the rest in steam.
As the piston pushes water through the HX, it comes back to the base of the group and through a single hole over the dispersion block. At this point, it is just like any other Gaggia with the same block/screen/screw/basket and a compatible portafilter handle (it's a bit heavier).
I found that you need to flush some water out of the HX prior to pulling your shot as it overheats. However, despite obvious signs of burning on the edges of the puck, I also found that it was wise not to flush too much, even to the point of the initial water being rather hot.
The Achille was more tolerant of a high dose in the basket, even requiring a moderately high dose in my experience. You can really stuff it if you want to. N.B. After filing down the rim of the shower plate on my Espresso, I find that I can pile on the coffee with the pump machine as well. Thing is, it is the same shower plate in the Achille.
Colin, thank you for a most informative and well written article.
From what I can see, Gaggia has rewritten the design book for all lever machines with its new Achille model. This new generation machine boasts several safety and technical features not found on older designs. For now, the Achille would seem to stand out from the competition in this price range.