> it is more proven than these other things being talked about. [... ] it seems
> much more established to me than this other low-wattage, small stuff.

With "other things" you include the PowerQUICC II Pro? If yes, let me tell you that it's massively used in the mid-range embedded network and storage space. In those domains it's a quasi-standard CPU. It can't really be proven more. I think there's magnitudes more devices using PowerQUICC II Pro than devices using MPC5121e (and probably even MobileGT in general). The difference is that really being embedded, in most cases the user simply won't notice that the device is driven by a PowerQUICC II Pro at all.

"PowerQUICC II Pro is used as networking processors for routers, switches, printers, network-attached storage, wireless access points and DSLAMs."
http://en.wikipedia.org/wiki/PowerQUICC#PowerQUICC_II_Pro
(note that MPC837x and thus the 800 MHz capable variants are missing from the article)

> I'd like to know how far performance can be pushed on slow, low-wattage
> processor by [...] dedicated graphics chip, like PowerVR

With the MPC5121e there's no need for a dedicated PowerVR chip because the SoC has a PowerVR core on-chip already. The discussed driver problem for the PowerVR core wouldn't magically be eased if that core was additionally put to a dedicated chip (resulting in two unsupported cores).