MacBook Pro 13" mid 2014 at 60hz 1440p scaled with Dell P2715Q

After extensive research I opted for a Dell P2715Q to use at work with my 2014 rMBP 13".

The problem is I can't get any acceptable display resolutions!

I'm using the enclosed Display Port to MiniDisplay Port cable. Ideally I'd like to use 2560x1440p 60hz scaled with hiDPi so that I can get somewhere close to the Retina quality I'm used to. I only seem to be able to get 4k at 30z or 1080 at 30hz (chosen from display settings within OS X) Yosemite 10.10.3 Either of these are only options using MST selected on the monitor settings (and according to the onscreen Dell menu both of these options are 3860 x 2160. I am aware that my rMBP doesn't support 4k at 60z which is fine because I'd be happy with a nice sharp 2560 x 1440p 60hz image and it seems lots of other rMBP owners are using this option so why can't I?!

Also when switching to MST mode on the monitor the only 60hz res I can get is 1280 x 720 which whilst it looks sharp and smooth is far from ideal as everything on screen looks huge! All the other hiDPI options are at 30hz are horrible. The only 2560 x 1440 resolution I can run at 60hz is low resolution and is awful. It seems I can't have a nice sharp picture and smooth operation at 60hz together. HELP!

I'm really at the end of my tether and the only solution I can think of is returning the monitor and buying a 2560 x 1440 monitor with a decent DPI or selling my rMBR and getting a 2015 model.

Does anyone have any bright ideas to solve this problem? I've even tried using SwitchResX to no avail.