Ok, this is what I’ve tested so far In SFA3 arcade version with A-Ryu + A-Ryu :
c.FP : 16 pixles
red mashed : 8 pixles
close FP Hadoken : 12 pixles
red mashed : 6 pixles
close FP Shoryuken : 19 pixles
red mashed : 10 pixles
3-hit RH Tatsumaki Senpuu Kyaku : 15
red mashed* : 11
lv.3 Metsu Shouryuu Ken : 23 pixles
red mashed** : 12 pixles
close lv.3 Shinkuu Hadoken : 55 pixles
red mashed*** : 40 pixles
close lv.3 Shinkuu Tatsumaki Senpuu Kyaku : 62 pixles
red mashed**** : 59 pixles
- I can only mash the first and third hits. Seems impossible to mash the second hit.
** I sucsessfully mashed all 3 hits of this move.
*** I looks like I flashed at least twice. This is normally a 5 hit move. I am assuming it’s impossible to mash all 5 hits.
**** I was only able to red flash on the first hit of this move, it seems impossible to mash the rest of the damage down.
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blocked c.FP : 10 guard pixles
blue mashed : 5 guard pixles
blocked FP Hadoken : 2 life / 4 guard pixles
blue mashed: 2 life / 2 guard pixles
blocked FP Shoryuken : 3 life / 9 guard pixles
blue mashed : 3 life / 5 guard pixles
blocked 3-hit RH Tatsumaki Senpuu Kyaku : 3 life / 6 guard pixles
blue mashed : 3 life / 4 guard pixles
blocked lv.3 Metsu Shouryuu Ken : 5 life / 10 guard pixles
blue mashed* : 3 life / 6 guard pixles
blocked lv.3 Shinkuu Hadoken : 10 life / 13 guard pixles
blue mashed** : 8 life / 5 guard pixles
blocked lv.3 Shinkuu Tatsumaki Senpuu Kyaku : 13 life / 21 guard pixles
blue mashed*** : 13 life / 20 guard pixles
- I was able to blue mash all 3 hits
** I was blue for the entire block sequence. No idea how many times I “flashed”.
*** I flashed blue on every single tick.
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To me, it looks like red mashing is strait up 50% on all sucsessful flashes.
It also looks like strait up 50% less guard damage with blue mashing. Except for the Shinkuu Tatsumaki Senpuu Kyaku. Blue mashing doesn’t seem to effect this super. It also seems to be impossible to red mash anything but the first hit of this super. WTF is up with that? 
It also looks like all calculations are math_ceil. Which means that it always rounds UP. Thus, you never take 0 damage.
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It appears that any directional button and/or any strength attack button may be used for both red and blue mashing (start not included).
[Edit]: Counter mashing.
Aparently it is possible to counter-mash. If you are mashing at the same time your opponent is mashing, who-ever gets hit or blocks will be unable to reduce damage/guard damage via mashing.