Is my GPU bottlenecked or is Borderlands 2 not very compatible with AMD system?

My specs are as followed;
-Athlon II x4 750k Black Edition 3.40GHz
-Sapphire R9 270 2gb
-Corsair Vengeance DDR3 1600Mhz 2x4GB Dual Channel 8GB kit
-500w Corsair Bronze
-1TB WCD Hard drive
-Gigabyte GA-F2A88XM-D3H Motherboard
-Windows 7 Ultimate
-BitFenix Comrade Case Midi-Tower

Hello

I have played Borderlands 1 and now i am playing the second one. I am having minor fps loss in big battles and major fps loss to 30-55 when looking down to a long distance of buildings. I have disable dynamic shadows in both forced off in the config files and i have force the Physx off. Which has given me a boost in fps except for the long distance fps loss which i only experience now and then.

Would this be with my cpu, i doesn't got to full load though. I'm looking to upgrade my CPU to a Intel Core i5 i5-4690K if i have to but i don't know which board to get. I'm just looking for a basic board like my current one. I'd like to stick to the same manufacturer as i like their boards.

Thanks
 
Solution
I strongly think the CPU is holding back the GPU'S performence or bottlenecking it so overclock your CPU to a good level would help and if you upgrade your CPU to i5 4690k that will be much better .

For i5 4690k you will need a z97 chipset board any z97 chipset board will work with i5 4690k.

http://www.amazon.com/ASUS-Z97--DDR3-2600-Motherboards/dp/B00K2MAU5Q/ref=sr_1_1?ie=UTF8&qid=1411942824&sr=8-1&keywords=z97+motherboard

http://www.amazon.com/Gigabyte-GA-Z97X-GAMING-Gaming-Networking-Motherboard/dp/B00K2RQDXY/ref=sr_1_5?ie=UTF8&qid=1411942824&sr=8-5&keywords=z97+motherboard

http://www.amazon.com/MSI-DDR3-2600-Motherboards-GAMING/dp/B00K23BZVI/ref=sr_1_2?ie=UTF8&qid=1411942824&sr=8-2&keywords=z97+motherboard

Few good z97 board to...