Pre:
LeBlanc can cast the previous spell she cast. The mimicked spell deals 10/25/40 % increased damage.
Post:
LeBlanc can cast the previous spell she cast. The mimicked spell deals 125/200/275 (+0.2*AP) ADDITIONAL damage.
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Pre: QRE at level 6
[Q] 150 (+0.6)
[Q proc] 60 (+0.3)
[R] 165 (+0.66)
[R proc] 66 (+0.33)
[E] 40 (+0.5)
[E proc] 40 (+0.5)
Total: 521 (+2.89)
Post: QRE at level 6
[Q] 150 (+0.6)
[Q proc] 60 (+0.3)
[R] 275 (+0.8)
[R proc] 60 (+0.3) <-- I'm assuming nothing changed when you proc R
[E] 40 (+0.5)
[E proc] 40 (+0.5)
Total: 625 (+3.0)
So once you hit level 6, you're doing a significantly greater amount of damage (104 (+0.11) is about 20% more than you normally do) more with your full combo assuming you use W to gap close and not do damage. But that's not all! Your QR poke alone does about 25% more damage than before because the damage from the mimicked Q is frontloaded (i.e. the proc on R does less damage because the 10%/25%/40% scaling is gone!)
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Pre: QRE at level 18
[Q] 230 (+0.6)
[Q proc] 100 (+0.3)
[R] 322 (+0.84)
[R proc] 140 (+0.42)
[E] 140 (+0.5)
[E proc] 140 (+0.5)
Total: 1072 (+3.16)
Post: QRE at level 18
[Q] 230 (+0.6)
[Q proc] 100 (+0.3)
[R] 505 (+0.8)
[R proc] 100 (+0.3)
[E] 140 (+0.5)
[E proc] 140 (+0.5)
Total: 1215 (+3.0)
Even though her ratio with this combo has decreased because the new R gives a flat additional +0.2 per AP, to make up for the difference in base damage alone (1215-1072=143), you would need almost 900 AP. So basically if you execute a QRE correctly, you will do more damage with the new changes up until you have 900 AP (which you will probably never have anyway!) Assuming you have 500 AP, you will still do approximately 5% more damage than before by executing and proccing all parts of your QRE correctly.
Finally, with the flat damage and flat AP ratio on R, you will never have to choose between mimicking Q and mimicking E no matter how much AP you have. It is always better to QRE than to QER with the new changes.
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TL;DR: Leblanc does anywhere from 20% to 25% more damage once she hits 6. Even at level 18, despite flat AP scaling, she still does around 5% to 7% more damage with her combo.
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it's a buff unless you have 900 or so AP basically.