Yeah, water isn't compressible but I'm pretty sure that given enough pressure, pex can bulge a little bit (kind of like a really long thin water balloon), so I was thinking that maybe the pex line was doing some of that. More line == more surface area .. etc. etc. blah blah blah. (I work for a pipeline automation company remember?
Oh right, you Metso guys sold us off, we were making too much money or something and it was making the rest of the company look bad. J/K!!
)
Anyways the shutoff part works fine now (there is a lag though between float valve shutoff and solenoid shutoff) but now the really annoying thing is that the solenoid reopens about every 5 minutes for about 30 seconds or so and every time it happens it sounds like someone having really bad gas ("EeeeeeEEEEEyyyuurrrrrrRRRRp!!!!!!"
That wasn't me, that was my RO unit. Honest! 
) .... and you can hear it throughout the entire house. I've tried adjusting the throttle a bit but it doesn't really change anything except for the frequency.
I don't know... I have about 30-40' of 1/4" pex and maybe that is an inappropriate tubing to use for this application? Should I think about stepping it up to 3/8" or something? I don't really see how the increased diameter would really help with this phenomenon, though, now I'm starting to run out of ideas.
For now I just turn the unit off if it gets too annoying, which seems to work OK so long as I remember to eventually turn it back on (the reservoir servicing two tanks is only 20g, but the powerhead pressuring the tank lines sits about halfways up the reservoir (I hope to eventually make this a sort of kalk reservoir) and it doesn't take more than a day and a half or so to replace 10-15g of evap from the two tanks).