The quality of the source that comes from tv broadcasting is pretty poor so the benefits of using external speakers is not that great compared to well engineered/produced CD's, Blu-ray movies etc. Source/Quality of recording > Speakers > Room. I notice a lot more sibilance with voices with external speakers which bothers me. I don't have any trouble hearing voices with my tv using the built in speakers so I never have to turn it up loud. That is the problem with tv speakers is that they will compress and sound nasty when turned up. If you have to crank your tv sound up you will find benefits from external speakers.
The only time I enjoy multichannel sound for tv programming is with sports in HD and even then the mics can be mixed hot and speech doesn't sound just right.
