Ok just got home, so I can give my thoughts a bit more extensively on the changes. I'll preface this post by saying sims for ToS will be out once the 2pc/4pc are finalised in about 2-3 weeks from now. I'll do legendary comparisons and trait rankings then. For the moment, let's focus on the three major bits of information available: the Starsurge/Starfall changes, the loss of T19 4pc and the gain of T20 2pc/4pc.
1. Information Available
Balance
Stellar Drift now increases Starfall’s damage by 50% (was 60%).
Starsurge damage increased by 11%.
Tier 19 2pc: Solar Wrath/Lunar Strike 15% crit
Tier 19 4pc: 15% Starsurge damage on targets with both DoTs
Tier 20 2pc: Increase max AP by 30 and increase Starsurge critical damage by 30%
Tier 20 4pc: Starfall/Starsurge increase haste by 3% for 15 seconds, stacking up to 5 times. Mechanic: metronome.
The rest of the changes just look like tooltips are being changed to reflect the 4% buff across-the-board we received in 7.1.5.
2. Damage Analysis
So let's unravel these changes and see what's happening. I think most people are concerned with the Stellar Drift nerf, so we will start there.
Starfall damage 7.2 = S*1.6
Starfall damage 7.2.5 = S*1.5
% change = (1.5-1.6)/1.6 = -6.25% Starfall damage
If we assume Starfall is 30% of our AoE damage, then:
% change (AoE damage) = -6.25%*30% = ~-2%
So the Stellar Drift nerf is about a 2% AoE damage reduction, due to a 6.25% Starfall damage loss. Not that bad, but AoE nerfs feel a bit bad considering we don't do that much AoE anyway. Moving on: Starsurge.
Let us assume that you really like single-target damage, so you have 4/4 Starsurge crit traits, you picked up two Starsurge crit relics, so you have 6*8 = 42% extra crit to Starsurge. Let's also assume you have 18% crit passively, so your total Starsurge crit chance is 60%.
Starsurge damage 7.2 = SS*1.15 (from T19 4pc)
Starsurge damage 7.2.5 = SS*1.11*(1+0.3*0.6) = SS*1.31
% change = (1.31-1.15)/1.15 = 14% Starsurge damage
So assuming you have the set-up as indicated above, your Starsurge damage should increase by 14% when you swap your T19 4pc for T20 2pc. This doesn't factor in ED, getting three Starsurge relics, the max AP increase from 2pc, nor the T20 4pc. tl;dr -- Starsurge damage will be nuts in 7.2.5. Also note that CI is still relatively new and will amplify this effect.
If Starsurge is 30% of your ST damage, then this is a 4.2% damage increase alone. When you factor in the things that I didn't here, we should see our ST damage being very competitive in 7.2.5.
I think that's about all I wanted to contribute for the moment. Once simc is updated I'll give a lengthier post on legendaries and traits.