Any custom part can do a specific job more efficiently than a more generalised part, but you are trading performance for flexibility, and the past 20 years have shown less and less specialisation in favour of more flexibility. PS3 features the last GPU from nVidia to have specific vertex and pixel units, with nVidia talking at the time about how discrete shaders are more efficient yadayada, and look where that got PS3. I can't think of any discrete part that'd make sense in the consoles' silicon budget. Video decoding, data encryption/decryption, physics, raytracing, are all jobs served by discrete chips but which don't make sense bundling them all together like a 21st century Amiga!