Hey,
Both the Mad and Alpha's do leak significantly more than the p7's.
But far less then the LCD2's I used at work for a while
(quietly, which was a real shame, the LCD's were the first headphone I have owned that I felt stayed completely clean and true to its sound far beyond comfortable levels).
I honestly dont know what planar speakers sound like. Like dynamic headphones, they have different signatures from HifiMan to Audeze to Fostex, however of the ones I have tried I would say the similarities are deep base extension with fantastic texture, lush deep sound (not thin sounding, plenty of body), mids are divine, again lush and of the ones I have had, very detailed highs without being overly strident or harsh.
In saying that, I hear the HiFiman 400's can be very harsh up top. Anoher thing to consider is all the planar's I have had/heard were heavy, which was one of the only things that lead me to sell them due to a bad neck.
I couldnt recommend the LCD 2's/alphadogs more for sound. The mad dogs are the lightest of the bunch and also the most comfortable to me. Not to mention most affordable.
I have high praise for the Mad Dogs Bang for Buck.