While searching on how to seat lower in my dc2, i found people saying you could increase the space between your head and the roof by as much as 2 inches with aftermarket rails. I took pictures from before (seating in the gsr leather seat) and after (seating in a dc2 recaro with bride type ro rail).

Before (gsr leather seat):


After (dc2 recaro with bride type ro rail) :


I wouldn't say the space increased by 2 inches, but it certainly increased by at least 1 inch (which is just enough for me to put my helmet and stop hitting the roof with it).