also true, at least for the older MH systems; they took around 5 minutes to start, and even if you turn them off for a second, relighting them takes another 5 minutes. i think there are newer ones that light off faster, but still not like electronic ballasts in modern fluorescents (ie, instantly).
the heat is an interesting point. not sure if 6x 400w MH fixtures would throw more heat than 40x 4' two bulb T8 fixtures.. at least in my case though, one of the pros of all the fixtures is i'm able to 'zone' the lights and only use what i need. with six fixtures running down the ridgeline, i'll be able to just flip those few on and have enough light to navigate the entire structure while drawing about the same power as ONE of the MH units. then each of the 6 20'x20' bays will have 4 more fixtures, plus extras over the workbenches etc, all switched separately.
ahm