I have a Thermaltake Massive for my G752VT, and I dont feel any significant change on the temperature, I think that just feel cool if you want to use it on your lap
The vents under the drive bay and memory area are the intakes and part of the design to pull air across those components. As mentioned the coolers will give you a buffer to keep the bottom open but I dont think they would make any real difference. My G752VY temps at full load are perfectly acceptable.
“Two things are infinite: the universe and human stupidity, I'm not sure about the former” ~ Albert Einstein