Live with the high temperatures. I've never found a good solution.
As for lighting, I use 4---6 40W bulbs about 16" (40cm) from the pots.
While the number of bulbs and power sounds more impressive than your
15W bulbs, mine are much farther away. As you know, the intensity of
light will drop off more or less as 1/R, "R" being increasing distance.
I prefer using more powerful lights and a larger light---plant distance so
it is easier to enjoy them.
The tubes *can* become fairly warm, especially as the bulbs age and become
less efficient (my unproven impression). But most of the heat is produced
by the ballasts. I remount the ballasts on all my plant bulbs.
Note also that the amount of light emitted fluorescents decreases by a
large amount after several months. I buy lights and use them in the
terraria. Then after several months I replace them with new lights, moving
the old lights to less critical areas like the work shed, etc.
By keeping your lights reasonably fresh, remoting ballasts, and keeping
the CWC (Cheesie Wardian Case) from other heat sources, you should get a
temperature drop. I have also at times put plants on a nocturnal
schedule. That is, I illuminated them at night when the ambient
temperature is coolest, so the terrarium didn't heat up as much. Of
course, you can't do this in the bedroom.
B