In my experience, the Model Color works just fine when appropriately thinned (and mixed).  I do use the Flow Improver, but I can't swear that it makes a big difference.  As per the late Bill Welch, you want a big needle for acrylics (0.5 mm or larger), and a modest air pressure of 20 psi or less.

The Model Air paints are ready to spray out of the bottle, but I still add a few drops of the Improver.

I like and use the Vallejo paints quite a bit, but feel that they are a tad short of the mark when it comes to replicating Pollyscale.  YMMV of course.
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Dave Parker
Swall Meadows, CA