I have just take a look at embermage thunder skills. I previously played only ice
I have realized that Thunderstorm is the only skills without tier bonuses.
One of the great thing about tier bonus is that it makes leveling more fun because you have something to look forward to every 5 skill points (more than just increasing damage and proc chance etc.)
Maybe just a simple bonus such as more bolts released or remove physical damage reduction and give it as a bonus for higher tier something like that might be nice.