I am currently using a 4x 32W electronic ballast, and even when connected to four T8 32W bulbs, it gets only warm, no problems touching it. One thinks that with T5 technology being more advanced and efficient, the ballast would not get that hot...
What made you choose 28W pairs instead of single 54W HO per fixture?
Here an article that "sheds some light"
http://www.lightsearch.com/lightnow/technology_T5.htm