Maxed out everything except the star multiplier (that gets pretty insanely expensive at higher levels - but it doesn't really matter since by the time you're close to even trying to max it out you've already bought all the star upgrades anyway).
Never figured out what the "pressure energy" was supposed to be. I got the impression from the slowly growing number on (what I assume is) the pressure power indicator that it provides a multiplier on the pressure power, but the number of cubes/second processed was always just equal to the purchased pressure power, not the number displayed in the factory UI. A bug in which number is used in the cube-subtraction loop? Or am I unclear on what the grey number under the Ready/cube counter is and what the pressure energy does?