Is R9 290x overkill for 1080p gaming?

yes it is overkill. however not every gup with the same specs works as well in terms of fps for the same game. Every game is different. not just in terms of quality, the amount of rendering needed etc but actually how each game uses the cup and gpu. for example the elder scrolls online because it is cross platform (xbox 1 and ps4) as well as on pc, the game is optimised to use 2 cores of the gpu (as xbox 1 and ps4 only have 2 real cores). However stupid bethesda have placed 100% of the 3d model rendering for the elder scrolls online on one gpu core. they did this because the consoles have fast powerful cores designed for gaming. and the game runs fine for them. however this means that for pc, no matter if your gpu had 100 cores the game could only use 2. so if you had a hundred gpu cores that were weak as compared to 2 or 3 super powerful cores, you were stuffed. in big fights with a lot of players, there is a ton more pressure on the gpu to rendered things. this caused many beta testers for the elderscrolls to experience massive fps drops while in big fights, and it wasnt their gpu, it was the way the game was made.

in the same way, every game uses the gpu and cpu differently, and this ultimately affects the amount of fps your game will have. going back to your question, yes your gpu is killing it. it is hard to compare to how well it will work for every game because each game is made differently as seen with the elder scrolls example. but for 1080p gaming for GTA and battlefield 3 (not sure about hardline havent seen the benchmarks) you will get well above 30-40 fps. more like 80-90.

however your gpu isnt bottlenecked by your cpu as you have a very good one. you will be able to play almost every game with high fps. if you can afford the 290x definitely get it, as you are future proofing for the rapidly increasing demand that games are placing on computer tech.

what i mean by all this is, you have a great cpu. getting a lower model gpu is DEFINITELY fine for the games you want to play and certainly great for any other demanding game. however in heavy model high quality mmos particularly, or with first person shooters that have a lot of models, you will experience a drop. a drop to around 30-40 fps? probably not depending on the game, but a drop indeed.

get that gpu. it works well with your cpu and ram. you are set to go for almost any game you want. but you can definitely get away with a much cheaper gpu for those games at only 1080p

also with your part compatiability and cpu fan, yes you will need an aftermarket fan if you plan to overclock. and your cpu is designed to be overclocked (The k means the processor has an unlocked multiplier, meaning you can overclock the CPU in the bios by simply increasing the CPU multiplier). overclocking increases the cpu power, but also increases temperature, therefore the stock fan probably wont be able to keep it to nice temperatures if you want to overclock a lot.

hope i helped