i've been playing with 'grey screen' material as well recently, and though i haven't gotten samples of the firehawk, i did notice that NO grey screen material looked any good at all in white scenes next to a normal white screen. however, when i set up a full 'grey screen'(i used a blanket that was actually super dark) my eyes quickly adjusted and the whites looked acceptable again.
the thing with grey screens is that no matter what the gain is, there is no amount of light you can project on a grey surface to make it look white. just like there's no amount of blue light you could shine on a red surface to make it look blue. but our eyes do adjust, and when you don't have a true white source to compare to, we start to perceive the light grey as white.
i'm trying a cheap grey screen, ordered it a couple nights ago, and i'm hoping for the best. but honestly, even though i'm all about the blacks, i am a little concerned about the grey. it seems as though it's either not going to be dark enough to really improve my blacks much, or it'll be so dark that there's no way the whites will 'pop'.
fortunately i get to play trial and error with my parents theatre first, haha. and they are so far from picky they'll be happy with anything(including the bedsheet they've been using until the screen arrives).
if you have the option, i would strongly suggest viewing some FULL screens(even if it's not exactly the firehawk) in a store somewhere to get a better idea of how they look.
Displays: Samsung PN64F8500/JVC X35
AVR: Pioneer VSX-1130K, 7.1/5.1.2 audio
Sources: HTPC(Enby), PS3, XBOX360, Wii
Control: Harmony One