First off it's important to note that proper empirical testing of typeface performance remains elusive. So we don't really Know too much. But that doesn't relieve us of the need/benefits of thinking, and acting on our conclusions.

Contrast: Do you mean a difference between thick and thin parts of letterforms? If so, it's treated (by designers who think straight :-) as its own thing, so others factors don't have to affect it, at least not one-directionally - it's more a matter of the various attributes being in balance.

Flaring: I'm assuming you mean where the ends of "strokes" get thicker. Although serifs are agreed by most people as helping readability, and flares could be seen as "pseudo-serifs", I suspect most people incorporate flaring as a purely stylistic element, not a functional one.

hhp

Hmmm... i thought flaring meant getting the edges thinner. As you can see in the page 11 of the spec book of arrival sans(http://keithtam.net/documents/arrival_spec.pdf)...
According to the spec books he says flaring increases the visibility from a distance. Is this true?

Tam is also using "flaring" to mean making the ends thicker.

hhp

I have poor contrast sensitivity and read using a bold font, one with thick strokes, e.g., Arial Black.
Fonts with serifs tend to have thin strokes connecting the verticals and my eyes don't see them, leaving a forest of tree trunks with no branches, so I stick to fonts with uniform strokes as being of higher contrast throughout.

On the other hand, when displaying copy in white on black, I use a font with thin strokes, the copy standing out and my eyes not being blinded by the blaze of white in a b/w display.