Registered for this site specifically for this thread, because many of you are quite wrong in your assumptions.
They can overclock the laptop hardware just fine, the alienware cooling solution handles overclocking very well. I have an Alienware M17X R4 custom modded and heavily overclocked laptop and the system handles the overclocks very well.
Example: The Alienware 17/M17X R4 cooling solution is a dual fan 2 heatpipe CPU heatsink design, and a triple heatpipe large spreader GPU heatsink design(which is also in contact with the RAM.) They are direct copper contact designed and have quite large fin arrays.
The alienware 18 has the same dual pipe CPU heatsink(different design) and dual triple pipe gpu heatsinks.
There's plenty of thermal headroom for overclocking the CPU and GPU's in an alienware 17 and 18.
My Super Aienware M17X R4(modded) has astonishingly good thermal performance, with the GPU topping out at 67C with nothing but sitting flat on the desk, no cooling assistance, just its built in feet.
CPU tends to hang around 75C~80C reguardless of the overclock.
My system OC'd parts: i7 3740QM at 4ghz, HD 7970M 927mhz Core 1225mhz Memory 1.008v Core Voltage. Max termperature I've seen is 67C.
Link to my build:
http://www.overclock.net/t/1397732/my-alienware-super-m17x-r4-custom-build-log/0_100
The real question is what the overclocking limitations are on the i7 4800MQ on the hm87. the i7 3740QM is maxed at 43 41 39 39, and depending on how much you wanna mess with your Bclk.