For accuracy the paint to barrel match is very important. The Freak system is the most versatile and cost efficient. Of course, if money was no object you could buy one barrel for each diameter of paint you may encounter.
In terms of efficiency a non stepped barrel is a bit more efficient than a Freak type system. I don't believe there is any major difference, meaning that a Freak system is not a gas hog as compared to a non stepped barrel. Just a difference in efficiency.
When you are trying to figure out "flow" - constant and greatest as you referred to it there is not much to consider these days. Most of the manufacturers would have you believe their system is the best. The fact of the matter there are no real bad ones. In fact, I have yet to see any type of empirical data to back up any of their claims that one brand is better than another. I had an Air America Apoc and sold it. I now use Crossfire screw in bottles and they work just fine for 1/3 the cost of the adjustables.
Well there you have it...just my 2 cents.
Hope this helps,
Excaliburs #156, #304