You could be right, it's innards a bit better than M2, but so much better it warrants twice the price? Personally I have abx tested FLAC against mp3's and cannot ascertain the difference, so I'm rarely convinced of any High Res argument.
Powerwise I'd expect fine for yer IEMs, perhaps not for yer cans. The vast majority of size of that Plenue D will be battery, not amp components. If yer looking to drive yer cans adequately then I would recommend something more along the 'boutique' line of DAPs, you know... the ones that give out only 10hrs battery life etc as they seem to have the components inside built for powering cans better.
I'm not a can lover, tho I do own a couple of pairs, and my Hidizs AP100 certainly drives them better than my Cowons. I'd expect Cowon's M and P1 to do the job better, but not all cans are made equal, it maybe that those specific ones you mention will be fine.
What I have learned is that cans vary enormously in terms of sound quality depending upon ampage. I own Samson SR950 Studio Monitors, they are incredibly difficult to drive, connected to say just a Fiio E7 amp makes it sound better, but when using in the Fiio E7+E9 USB DAC dock combo it's another thing entirely, the cans really open up and I hear the full potential of them (which makes sense as being Studio Monitors they are intended to being connected to higher sources that what usual OTG setup can give).
My point being that you may always be 'unsettled' as to what you might be able to hear from the cans if you had amped, and indeed even those users who buy boutique DAPs still amp ontop of that to drive their cans sufficiently. Sadly to find out if that is necessary for you might result in you buying a separate amp and testing.