Some tables I worked on in excel to calculate the approximate value per HP point in upgrading mounts vs. runes.
Edit: Since the forum wouldn't properly utilizes spaces to ensure the table had the values under each header correctly, I took a screenshot of it instead.
Attachment 882
So you do have a point about leveling the tiger to 7. Mathematically speaking spending gold (gems are another story) on mounts above level 6 is less efficient than leveling an endurance rune to 12. Remember mounts give HP to all heros, so that is 4000 each, and assuming you have a sub ~ the total HP gained per level for a tiger is 4000 * 6, which equals 24000. I will do calculations for the bear once I get one.