Wow, you've really perfected the heat dissipation plan!! I can think of one reason to get the 7 channel amp if it is not actually needed. That is to bi-amp the front left and right channel for stereo music enjoyment. That would only be of value potentially, if any, for large passively bi-ampable speakers that have the tweeters on it's own or the mids and tweeters together but has woofers that are big and extended down low to 20 Hz.
Here's the math:
2 channel, $999.9 ($499.95/Ch) - 400 kVA/Ch
3 channel, $1099.9 ($366.63/Ch) - 340.67 kVA/Ch
5 channel, $1299.9 ($259.98/Ch) - 512.5 kVA for the 2 ch group, 410 for the 3 ch group
7 channel, $1599.9 ($228.57/Ch) - 340.67 kVA for the 3 ch group, 310 for the 4 ch group
Based on transformer size, I would definitely go with the 5 channel version. Note that without seeing the schematics, the transformer/channel grouping above is just my educated guess. If you want to know for sure, email ATI, or may be Gene has the information.