For the same brand of bulb, HQI
does put out more light per watt, and conversely generates less heat per watt. I've been told that this is a feature of bulb design, and has to do with the fact that current passes over the bulb coils in a single direction without having to be looped back like in a mogul. The end result is less resistance at the bulb and higher output.
To look at a "real" scenario, compare the XM 20k double ended vs 20k mogul bulbs. Using the same HQI ballast, the DE bulb generates more light. This demonstrates that the increase in intensity is a function of the bulb, not the ballast.
link
(sorry for the long link)
Edit: I have modified the link so that it shows up as smaller. If you click on it, it is the same URL, but this way it doesn't bugger up the thread width. -Tony