I've been dealing with this issue for over a month now. Think I've read just about every related post on the AMD Support Forums as well as elsewhere.
I have a 5 year old Samsung Series 7 Chronos Notebook with an i7 CPU, 8GB RAM, and an Mobility HD8870 graphics card. Running Windows 8.1 from the factory. The laptop comes with onboard HDMI and VGA only (no DVI). According to the spec sheet, it can support at least 2560x1600. I have a Dell UltraSharp 27 InfinityEdge which has an Optimal Resolution of: 2560 x 1440 at 60 Hz.
I've tried 5 different version of AMD Catalyst/Adrenaline driver going back from the original to the latest. I've even tried 4 different types of HDMI cable. Recently, I completely rebuilt the laptop and also dual booted it just in case there's a problem with Windows 8.1. I now have 8.1 and Windows 10. In both OSs, I am unable to get any higher than 2048 x 1080.
The suggestions about changing various configuration properties inside Catalyst are either outdated (options no longer exist) or do not work. Also tried a myriad of third party tools to force custom resolution. Nothing.
The same cables make it possible for me to get max resolution from a crappy $500 notebook with only Intel graphics running of HDMI. So what's going on? does lack of a DVI port mean that this although this chip can produce higher resolutions, I'm stuck with 1920/2048 x 1080? Something is going on and the AMD marketing does not live up to it's claims.