About 50-75% (75% with the 7970m, 50% with the 6970m as an example) GPU performance wise (with an AMD or Nvidia card not the CPU GPU which is a lot lower performance) and cpus in laptops are pretty good some are close to desktop cpus and some are only like 10-15% slower.
The above example I give is for a highend gaming laptops with a single GPU and the desktop again would be a highend with a single GPU.
Highend laptops now make great desktop replacements and just as fast if you get them with dual GPUs. You can look at a lot of laptops now and see they are much faster than only last years highend desktops in some cases. If you don't have a need for a desktop or the space they are great devices now.