You want the CPU to use GDDR5 as L3 cache? No. Just no. Caches need to have very low latency. GDDR5 had horrendously HIGH latency. Worse than regular ol' DDR3.A good ideea would be to slap 1GB to 2GB of GDDR5 in there over a 192-256 bit bus, so the GPU can use that as a framebuffer (think A10-8800K with built in R9 270) and the CPU part can use that for L3 cache.
I wouldn't say unseen considering the concept of a shared CPU and GPU with common memory was the goal all along with a very good idea of what the benefits are. Maybe unseen to the average person, not those in the computer industry for 20+ years.APUs are really interesting products and they are really evolving into something completely unseen. The Xbox One experiment and the X360 with on board SRAM gave excellent results in performance/efficiency, plus, they never achieved to build fast L3 cache and the obvious workaround is a larger cache to avoid cache miss and larger bandwidth even if the client is not going to buy that expensive ram kit 2400 CL 9.