How much is Iceland/Norway/Western Europe warmed up by the Meriodional Overturning Circulation?
To answer the numerical aspect of your question: Rintoul &amp; Wunsch 1991 advanced a poleward heat flux in the North Atlantic (i.e. that generated by the Gulf Stream) of $1.3\pm0.2\times 10^{15}W$. 

That number of 1.3 PW (PetaWatts) have been reused in several reviews on the thermohaline circulation such as Clark et al. 2002; hinting that this number is at least realistic (since I'm not a physical oceanographer myself it is difficult for me to judge otherwise). 

References:
Clark, P. U., Pisias, N. G., Stoecker, T. F., Weaver, A. J. 2002. The role of the thermohaline circulation in abrupt climate change. Nature, 415: 863-869.
Rintoul, S. R., Wunsch, C. 1991.Mass, heat, oxygen and nutrient fluxes and budgets in the North Atlantic Ocean. Deep Sea Research Part A, 38: S355–S377.