Well, mantle shows to be a big boost for weak cpus already so they can run with a much more powerful gpu without bottlenecking it as much. having said that, right now we have exactly one game and one engine that supports it. and no real word on whether or not it will take off.
I'd say an i7 is a bit overkill. an i5 4670k will run games just as well. if you really want hyperthreading, and you don't want to overclock, get a xeon e3 1230 v3 for around 250.00 and an h87 motherboard for around 130.00. You get a locked i7 at an unlocked i5's price.
I'd say you'd have to completely rebuild your pc however. a 7870 is not going to play any new game at ultra at 1080p at playable framerates, and an A8 won't be able to feed a more powerful gpu without mantel. You could try going with a lower end i5 4430 and get a gtx 770 but that is a hefty upgrade.