Thank you for your answer !
I modified the TextField to use Image's instead of QuadBatch and did some benchmarking.
For 500 Sprite's, each one composed of one Image (with alpha = 0.99999) and one TextField I get the folowing results :
For 1 character text:
Starling TextField : 6 FPS, 1000 DRW
Modified TextField : 30 FPS, 1 DRW
For 10 character text:
Starling TextField : 6 FPS, 1000 DRW <- stays the same !
Modified TextField : 18 FPS, 1 DRW
For 10 character text, flatten'ed Sprite:
Starling TextField : 13 FPS, 500 DRW
Modified TextField : 13 FPS, 500 DRW
In my case I won't have more than 10 characters per TextField so I think it's worth it.