The speed of the GPU in the N8 and other Symbian phones isn't the problem. video memory is.
The GPU in the N8 only has 32MB of memory, and 9MB is used by the default GUI of the phone. When writing games this is a HUGE problem!
The PowerVR core uses shared memory of the RAM.

The GPU in the N8 is designed for quick manipulation of video streams from a camera module, not really for games, otherwise it would have a lot more memory.

Also the graphic memory manager (GOOM) in Symbian is very simple. On most os-es a lot of swapping between video and ram is done automatically (causing severe slowdown when you use too much video memory). On Symbian you need to use the GOOM otherwise your application crashes when you uses too much video memory.

I would very prefer the PowerVR in N900 or iPhone 3GS to the Broadcom GPU in the N8.

Current OpenGl support in Symbian^3 devices is challenging, but you can make some good looking simple games. Without opengl there is no way you can make games that look as good as on iPhone.

No opengl in new Symbian devices is probably the nail in the coffin of Symbian application development. Games are a VERY big part of the apps market. Most mobile games are moving to OpenGL and Symbian will be left out.
But it probably isn't that surprising, Qt OpenGL support is not finished (Qt3D modules are being developed, but not available yet), and Windows phone does not support OpenGL, so Nokia probably does not have much incentive to stimulate OpenGL mobile development on their mobile platforms.