Although I have to admit I've never "Grilled" on it I purposely made my grates 5" apart on my UDS so I could conceivably "Grill" on it at some point. I might need some air intakes on the upper portion to make it work right though.
5" is what I'd go with too.
2004 Weber Performer GREEN
Orange Super-Duper Fast
Taylor Made 9deg. R7TP w/S-Fujikura 6
Rescued 2002 Weber Genesis Silver B Gasser
SJG Tamale Pot Mini Smoker (SJG was a rescue)
18" Gold (rescue) SJS (rescue)
and a Pyro FIL