This was also true for me on a 2012 MBP (on HDMI and DisplayPort iirc). My XPS 15 had a similar issue where turning on the GPU made it heat up and become loud

This was also true for me on a 2012 MBP (on HDMI and DisplayPort iirc). My XPS 15 had a similar issue where turning on the GPU made it heat up and become loud - actually the discrete GPU offered no performance increase because the heat made everything throttle....... sad! At least I could use an external monitor with that one though. It's just a limitation of the laptop form factor. My desktop has a 45 watt processor, same as the i7s, and it has a huge block of metal and a 120mm fan to keep it cool quietly (but still audible under load). There's no way to fit a 45w CPU and 45w GPU into a laptop and make it work.