Lightning rider talent is stacking infinitely. If you take the talents then reset them and take them again you retain the points from before and gain even more. https://cdn.discordapp.com/attachments/198633011514834945/575391232633602069/unknown.png Essentially if you had enough gold and wanted to get super high you could. Here is my % after maybe 5-6 times respeccing and hitting a decent pack of mobs in arena.
Here is a couple more times respeccing then hitting a small pack in a monolith. Getting about 1250% lightning dmg per hit on mobs