You might gain a little in minimum frame rate depending on the screen resolution/video memory usage, but little else if the CPU is theoretically maxed out with a lower performing GPU.
The 100% CPU usage would only be in certain gaming situations. Not all gaming requires intensive CPU involvement, since the rendering of most games takes place within the GPU <---> VRAM subsystem. In a scenario where the CPU is maxed out, you're dealing with either poor game coding - which induces stalling (bottlenecks) between CPU <--> RAM <--> I/O (harddrive), and/or the CPU is being tasked with a more intensive workload -example CPU physics, post processing, information needing constant updating and amending such as large game maps, game AI and dynamic objects.