I've been mulling this very thing lately myself.
One thing I would say, for sure, is that no 20,000 lumen system is going to be on par with a 400w MH. But if you bump it up to the 8 set of 54W 4' tubes, those units put out around 40,000 lumens and you have a closer comparison.
Consider that 400W MH bulbs are putting out somewhere in the same neighborhood in terms of lumens, but also kick out lots of heat which requires more distance from the plant. So you give up a little bit of efficiency in a trade-off for heat issues/proximity.
Bearing in mind that the number of lumens degrades significantly over distance from the source Im not so sure that a canopy wouldnt get more light from a 432w t5 fixture stationed 2" away over a 400w MH bulb 2' away. Y'know? Then again any HID is going to have better penetration. Hm.
If I had to go the whole grow with just one fixture I take the HID switchable. If using specifically for veggin in a seperate room, especially one with possible temp issues, I would go t5.
At this point its all theory in my head, as Ive not been able to speak from experience on this particular issue (yet).