I've driven 4 55w bulbs with the WH8. Did it for a few months only, now the same ballast has been in use for over a year just driving 154w total. I'm moved from 220w of 6500k PLL down to 150w 4000k CMH because I started using a smaller space for vegging. I really want to go back to PLLs for vegging, my plants were much tighter with less heat. Their compactness made flowering more effective as well with what I use (combination of PLLs and LED). I just have to figure out how to make a fixture for this different veg space as it has an odd shape, which is fine for the CMH but harder to do with the ~22" long 55w bulbs.
In any case, it would likely be better to go with 2 WH5s than using a WH8 for 220w. Another advantage to this is that if one ballast fails, you still have enough light to get you through until you get a replacement whereas you might be out of luck with just one.