It sounds like the boiler uses an immersion coil or plate to produce the domestic hot water. What you see is probably the best you'll get. To ensure the water gets hot as it passes through that coil, they often put a restrictor on it so it spends enough time inside of the coil to actually heat up. This limits the maximum flow. If you want a higher volume and pressure of hot water, or more stability to the temperature, I suggest you add what is called an indirect water heater. This looks like a traditional tank, but instead of using gas or electric to heat the water, it uses a coil from your boiler that treats it essentially like a separte heating zone. Depending on the size of the boiler and the tank you select, you can get unrestricted volume and pressure of hot water from one.
If you tried to pump more water through that coil, it would not come out as hot, since it would spend less time in contact with the heat transfer device.