Nah, that's the one downside. But the 2.6ghz i7 in the Mini will definitely be good enough for most cases.
Looked into it and it seems the 21inch imac has the soldered cpu, only the 27inch can change. But a desktop quad core i5 is still a very nice CPU (and I'll be honest, i7s are usually best for the designing rendering crowd while i5s are fine for gaming).
Alright so Optimus is a technology nVidia made for their GPUs. You need a system with an intel integrated GPU (such as the intel hd3000, hd4000, iris pro, etc.) to use it.There are two main aspects of Optimus. One is that is allows you to render the internal screen, meaning you don't have to purchase an external (OS X doesn't support optimus so youll need an external screen for os x use). The internal screen also puts a hit in performance because some bandwidth is being used to pipe back to the internal screen, so internal screen performance will be worse than external. The second part of Optimus is compression. You'll be on a PCI-e 2.0 x1 link and the compression boosts performance in the range of 100-300%.
Not gonna lie, the easiest setup would be something like my setup, a quad core mac mini, external display, and external GPU. This gives me more than enough power to play games to edit video and since it is an external display, I can run OS X and Windows 7/8 easily.