I do not understand your your 60 degree water temperature comment as I do not believe that your water is reaching 60 degrees. Please do not confuse core temp with water temp. Yes for every degree increase in water temperature you will see a one degree increase in core temp but this does not mean your water is the same temp as the core. Most tubing and pumps will not pot up with water temps in the 60s anyways.
As to your 5 liters of fluid, how are you gaining so much capacity by the way, you well see a increase in water temperature pretty quickly with that chip.
So first some assumptions,
That chip is going to generate 220 watt hours of heat
Your radiator with those fans can dissipate let's say 120 watt hours of heat at a 10 degree delta
If your system is 5 liters of water it will take roughly 5.5 watt hours to raise the fluid 1 degree
So now some math.
220 watt hour chip - 120 watt hours of rad = 100 watt hours of heat into the water.
100 watt hours of excess heat / 5.5 watt hours of heat per degree C = 18.18 degree heat rise in one hour
60 minutes / 18.18 degree heat rise per hour = 1 degree heat rise every 3.3 minutes.
So yes you will see a increase in your water temperature rather quickly.
Now please bear in mind this is a very rough estimation and does not take into account increased radiator efficiency as delta t rises but it gets the point across.
All of these calculations are based off of a starting delta of 10 degrees so you are probably seeing about am additional 10 degree heat rise pushing your delta to around 20 degrees. This means if your ambient temp is 20 degrees plus a 20 degree heat rise gives you a 40 degree water temp which is contributing to your high core temp.
Back to my earlier post, If you get better fans that can help dissipate heat better you will be able to maintain a smaller water delta and for every degree you drop the water temp you will see a corresponding degree come of of the core temp. I don't know exactly what fans where used in that review so I cannot give a direct comparison to your fans but I will guarantee that those 500 rpm fans are a major limiting factor in your setup.