Hi Trainer,
it will depend on the specificatioin of the existing and replacement boilers but I would suggest:
Pressure - this is nothing to do with the boiler. Pressure is related to the height of the boiler above ground and in some cases the incoming water pressure is also a factor.
Capacity - yes - IF the replacement boiler is a larger storage capacity then the existing one.
Economy - maybe. New boiler will probably be a little more efficient - but that may be negated if you have a larger capacity as there is more water to keep hot.
Ian