Why? Do you want this thing to only be as powerful as Vita? Even less? 14mm is thin enough. You people are too obsessed with aesthetics. With a thick plastic casing at 14mm, I'm not confident in this being more than 50% faster than Wii U.
Strictly speaking a thinner device can actually dissipate heat more efficiently (as the ratio of surface area to volume is higher). Hence why the MacBook can get by without active cooling; the entire body is a large thin heatsink.