I would not invest in athel. You already have kael six star? You only need one starter champion maxed.

To be totally honest - no I don't think you'll notice a huge difference, at least in campaign farming. Most of the talents that yield noticable results do so elsewhere - WM/GS for CB, the blue one that boosts TM when your allies drop low, the green one that counterattacks after losing hp, etc..

if you already got a fully scrolled kael i would suggest building up some support champs like spirit host diabolist and warmaiden 

could you show us you roster ?

Phantom, the scrolls on your Kael will not make a big difference in campaign farming. It will make a big difference in Minotaur farming, giving masteries to your other champs. 

It's random wich champ of the team gets scrolls at Minotaur, but, very rare for Plarium and this game, it's not totally random. Champs that allready have all masteries skilled don't get any scrolls. So you can bring Kael in your Minotaur team and direct the scrolls to other champs. If you want masteries for one champ fast, bring 4 champs with full masteries and 1 champ without - all scrolls from the Minotaur will land at this 1 champ.

You will see a difference in some content, yes, but not a gigantic, over the top difference.

I saw Ophe mention Warmaiden as a potential support champion to focus on. I would actually recommend Aleks for this role. He has the critical AoE Decrease Defense and his multipliers are insanely high, especially for essentially a free champion. So he brings AoE Dec Defense AND will hit very hard.

focus on building a small team of level 60's that can do dragon, gear will help more than masteries at this stage.

Kael is fine for campaign farming and early dragon, i would focus on getting someone levelled with drop defence next.