cool tubes were designed to be used vertically with 360 degree light all the way round in small spaces, and vent the warm air away, a brilliant invention, but not as a half pipe shining horizontally.
to use them horizontally and save space is excellent, but if you have space then use the correct equipment for the job, you must get as much light to your plants as possible.
and metal halide sucks.
never let any one fool you.
you get clowns on ebay that add little shields to the cooltube to make it look great for horizontal growing, that is, they pay say 10 bucks for the tube and add the shield in a few seconds that cost say 3 bucks and charge you 60 bucks, brilliant eh. the best lights, my fucking arse.
a little cooler than a shade though, as the cold air travelling through has less resistance, but not by much. as the air is forced by a fan.
a few pics from google :
