I just changed my GTX 770 into R9 290X. On clear installation of GTA 4 I have got almost the same FPS count on both cards (about 70 fps). But on moded game (realism, lords textures, icehancer) it is a big problem: Below you can see the both devices benchmark compilation:
min: 29, max: 57, avg: 42
min: 15, max: 36, avg: 25
Could you tell me what is wrong? In every game there is a significant increase of fps, but not in GTA...
Can you help me?