Combi boilers might have changed since I got mine 12 years ago, but when mine was installed they couldn't be combined with an additional pump as it's already a pressurised system. The installer did a pressure test on the taps to ensure that it was sufficient for a combi boiler to be installed.
I wouldn't be without the combi boiler, no waiting for water to heat up, no running out of hot water mid shower/bath, no oil tank and no immersion heater. Pressure wise it's definitely not as good as a pumped power shower, but miles better than a standard electric shower and I just have an adjustable shower head for a stronger jet when required. The pressure does drop the higher you turn the temperature, so if you like your shower scalding it might not be ideal!!