Precisely, I would like to know if booting windows 8 through GRUB can affect in any way the global performance (CPU/GPU benchmarks, cooling efficiency etc...) in both Windows and Ubuntu 14.04 ...

I have noticed a huge performance drop in both OS since installing Ubuntu in dual boot mode (I had previously tested both Os separately on the same machine). Even in high Temp (+99°C) the fans don't seem to speed up (thus I don't think it's a hardware issue)

I'm using an Acer Aspire 5755G with legacy BIOS. I suspect GRUB since I have overridden acpi_os=default to acpi_os=linux, which got the backlight running.