What differences have you seen when running an OpenGL ES based iPhone game on a device using an SGX chip... when the game was originally developed for a device using an MBX chip?
MBX is an older chip technology than SGX. SGX has pixel and vertex shading capabilities, making it far superior. you can read more here: http://en.wikipedia.org/wiki/PowerVR
Rendering-wise, you should see few visual differences for OpenGL ES 1.1 applications developed on the MBX series (at least I haven't). Of course, the newer PowerVR SGX chips have much, much better performance (and are tied to a much better series of CPUs as well).
One area in particular that gets a boost is vertex buffer objects (VBOs). On the SGX chips, these are hardware accelerated, where they weren't on the PowerVR MBX Lite. My application uses VBOs heavily, and its rendering performance increases by over 10x when being run on an SGX chip vs. an MBX.
For more on the specific differences between the chipsets, Apple has a nice in-depth walkthrough of their hardware capabilities in the iPhone development videos accessible via the iPhone Developer Program site entitled Mastering OpenGL ES for iPhone - Parts 1 and 2.