A 20% increase is a huge amount if it allows their upcoming APU's to max games at 1080p and keep over 30fps.
That is where the money is going to be at. Selling a metric crap ton of APU's for around $150-350 (no idea on prices yet just guessing) that allow many new PC gamers to get into the game. With game development in it's current state (held back by consoles) those with high end GPU's aren't going to notice as much benefit from mantle. Who this is really going to benefit is the low end consumers.
If it works well it is going to majorly hurt Intel and Nvidia both by eating up all their low-mid range sales. Why would you buy a $200 intel CPU and $200 nvidia GPU when you can spend around $200 bucks for the same gaming performance in one chip. If the upcoming APU's are even decent I'll be getting one just to play with it and have an HTPC on the cheap.
Exactly! Also, graphic chips before GCN will soon have to be replaced. The thing is that they will be replaced with mid to low end chips...and here Mantle comes to play. Markets such as China and India...are not high end markets... But they play games as well... (and sometimes actually buy them )