AFAIK there's currently only 1 JPG exploit ItW. So if your system is patched against it, putting image/jpg to the list of excluded content-types shouldn't hurt.
Of course, our default settings are designed to protect even those who don't have the patches properly installed.
About the speed - I'd say that unless your connection is really really fast (100mbit+), no slowdowns should be noticable. This is mainly thanks to the "Intelligent Stream Scan" feature...
thanks
Vlk