Does Heirlooms yield the XP bonus when the level range is exceeded?
Solution 1:
No - the level range on the heirloom items dictate the level range that the experience bonus is active. In addition to the statistics no longer scaling with your level, the experience bonus becomes inactive.
One popular trick to consider is that the level range is fully inclusive, so the experience bonus is still active until level 81 for level 1-80 heirlooms.