Hi Jackster,
I strongly discourage to use 2 layers. I would use strip lines.
1. with microstrip you'll get an impedance error between 10% and 20% from the expected value due to the fabrication process in PCB. while when you use stripline the error is usually around 5%
2. don't forget that during the EMC testing, you may be asked to do the radiated emission up to 6 GHz due to the high-frequency clock you are using. And stripline gives you a better EMC shield.
I had a quick look at your layout below and my suggestion is to increase the spacing between the GND copper and any controlled impedance traces.
For example, if you see your PCB, the first 2 differential tracks (from the top) you have one track near a large GND copper. You need to expect on that track to have a lower impedance of the nearby tracks.
also, don't forget that GND planes(or land) on different layers tends to oscillate if you don't have enough vias ( set the vias pitch at labda 4), in your case, at least one vias each 2.54 cm
Good luck,