What did you end up doing Kevster? After exchanging some emails with Jerry I'm really considering switching from T5s to MH. He sent me this information:
"T5 light/ 4 lamps of 2' = 8500 lumens, probably 15 square feet of growing area
400 Watt metal halide = 40,000 lumens, probably 20+ square feet of growing area
1000 Watt = 100,000 lumens
You have at least 5x the light energy for growing plants."
Which has really gotten me thinking about trying out a 400W fixture, just need to find a place that sells them...