Lets start at the beginning. the water pressure has zero effect on boiler efficiency or fuel usage. Replacing your tank will not make a difference although, draining it will solve the pressure problem. Water expands when you heat it and it needs a place to expand to. If there is no air cushion in the tank, the water has no place to go other than out the relief valve. If the house will not heat and you had the boiler serviced recently there is a chance that the technician put a nozzle in the burner that is too small. 180 degree water is plenty hot enough, though you could jack it up to 200 if the boiler is fired at maximum.