Alpha R2, CPU upgrade

Hello,

I've got an Alienware Alpha R2 from a year or so back that I'm looking to upgrade the CPU for, but I can't find too much information about the motherboard to know what all I can upgrade to without it causing issues. The computer only has a 180 Watt power supply so trying to stick to a similar processor at 35W. It's currently running an i5-6400T which is Skylake architecture on an 1151 socket, running at 2.2 GHz to 2.8 GHz. I'm confident that an i5-6600T would work as it's still on a 35W, and would provide 2.7 GHz up to 3.5 GHz, but would that be my best option? I see there is an i7-6700T which has similar stats to the i5-6600T, going from 2.8 to 3.6, but I'm abit stand-offish on going from an i5 to an i7, even though both are 35W on the power supply. 

Also, when I spoke to someone in a chat session with Dell who told me the chip was soldered on and couldn't be replaced however upon inspection it can be. Any advice about the chips would be great.