If i close my Macbook Pro laptop whilst it's connected to an external monitor of lesser resolution, will I have more CPU processing power as it's pushing fewer pixels?
Yes you will. If you look at this thread - the CPU / GPU can actually do enough work that other tasks are artificially slowed down by assigning CPU to the kernel_task so that it can idle and lower the heat being generated.
Lowering the pixels needed to be displayed by disabling the integrated LCD will allow all the processing to show the main monitor. The only downside is if your Mac needed the display open to cool off the system (the new MacBook for example have no blower so closed mode can impact cooling more than other models).