Ive run both load simulations and a QTP script to measure the rendering times of a client side app... so you get the best of both worlds.
Usability wise ... performance is now considered as important to the user experience of on line products and therefore should be taken into consideration with your buisness owners who are interested in the whole user experience.
There's a new way to do GUI performance testing and that is by using the cloud (Amazon, Rackspace, etc). Install your functional test tool on one image and generate X instances of it to run in parallel. www.capcal.com/CloudLabDemo.html shows how to do it easily, but you can do it yourself with some extra time an labor!