Do the GPU and CPU have separate fans and separate air inlets and outlets? If not, they would contribute to each others heat load, right?

Depending on what version of slackware/kernel you are using, you could fairly quickly try the Catalyst (fglrx) driver and see if that helps.

The open source drivers for the gpu, I think, are also starting/getting towards dealing with power modulation. You could search at phoronix for this topic and see if anyone else mentions it. I don't use laptop so didn't really pay attention but seem to remember the topic coming up either here or at phoronix.