Did you see Franlmatlh and Jackal's power there?Jackal had 600 and Franlmalth 800.So naturally seeing Natsu's performance against them we can assume his powerlevel was at Tartaros Arc max to max 1000.So even for DF it is not possible to close a gap this huge.
DF has been stated the strongest form a Dragon Slayer can achieve.It has been stated the power rivals that of a real Dragons and posseses the power to destroy anything
Here's the link of DF
http://fairytail.wikia.com/wiki/Dragon_Force
And what you said "Igneel's power infused with FDKM" that is the point.It was "Igneel's power" that made the difference.It doesn't make the form stronger itself though.
I didn't say tbe movie is canon.I said DF being mentioned the strongest form of a DS is Canon.
Wait when did Natsu learn any other Fire Magic other than Fire Dragon Slaying Magic?He only used Black Flame Curses.And seeing those flames aren't black it is DS Magic.Moreover the final fight with Acnologia is approaching so it will not make any sense for Mashima to make Natsu lose his DS Magic.
And why Natsu didn't use DF till now is because he didn't feel it necessary.DF has always been used against the final villain of an arc so most probably it will be used against Acnologia.