Re: turning up the HU volume.....
A volume control is (or should be) logarithmic because of the way the human ear perceives changes in SPL. At low volume we hear small changes, but the louder it gets the more change is needed for us to hear a difference.
So to answer your question, 1/2 volume is way less than 1/2 power, and without knowing the HU implementation of the volume curve you can't really predict it.
1999 Buick Park Ave Ultra
Head Unit: JVC KWHDR720
EQ: Audiocontrol EQL
Front Stage: Hybrid Audio Technologies Imagine 5.25
Sub Stage: In process (will be Boston G310)
Amp: Boston Acoustics GTA-704 4 Ch
Rear Fill: Boston SE953 6X9's