http://www.wowpedia.org/Mount
Firstly, talents like On a Pale Horse
do not stack with the guild perk. Since the guild perk is only 10%, you are basically just ignoring it and getting the 20% from your talent.
Secondly, you're doing the math incorrectly. Talents like On a Pale Horse give you a percentage
of your mount speed. If your mount goes 280%, you are getting an extra 20% of 280%. Your final mount speed is 356%, not 310%.
The same will happen if you buy Master Riding. Your final mount speed will be a total of 392%.