Yeah, 5 GHz sounds a bit high. But, 4.7 doesn't sound unreasonable.
14nm to 12nm saw a 200 MHz improvement. GloFo 12nm to TSMC 7nm should see a greater improvement, so 4.6-4.8 turbo sounds pretty reasonable.
Either way, a 15% increase in IPC plus a max turbo of around 4.7 GHz would be enough to...