Only reason for a sudden increase in restriction going into the CPU block would be too increase the pressure of the water hitting the copper plate, which I guess could increase cooling performance.
I guess it comes down to the pump being used whether the restrictor has a great effect or not, if its already providing enough pressure than there wouldnt any benefit to having it. And in this case, its a detriment as it decreases flow to the point where cooling performance is diminished.
I wouldn't worry about it, but keep an eye on it for anything strange. Maybe the extra pressure was XSPC's way of making sure stuff doesn't get caught in the fins on the copper plate.
Also, there is the possibility the 5°C difference is because you might have just mounted the block or applied the thermal paste better this time.