The size and distance of the coils ultimately limits the amount of heat you can receive from your slab. Where do you live? Have you done a heat load calculation to see if you are in trouble? If you are in trouble, you might want to consider augmenting with baseboards, PEX in the walls, or PEX in the Ceiling.
I'm not sure from your post what is going on with your water temp. What is your boiler temp?