Ok. I've just flashed Customized_FR on my gf Xperia M4 Aqua Dual with FlashTool.
So far, so good.
I've disabled animation from Developers Option menu and test with SBH20 bluetooth, and bluetooth works good.
MM 6.0.1 runs very well with M4 Aqua Dual, because 16GB of ROM it's ok instead of 8 GB of M4 Aqua normal version.
After install all apps, phone have 7,1 GB free onboard ROM. That's good.
First impression are: better memory managment, better fluidity, system is smooth, Sony stock camera app it's slow but works well.
As my habit, as her uses it I will update you with the impressions
Thanks to all for the information and the feedbacks
The quality of the speaker is now awful, and listening music with headphones (wired) is tragic. I'm downgrading to Lollipop...