So if the new pump puts out higher volume but the same pressure, there's no danger. Either the rest of the system can handle the extra volume, or it will choke it back. For instance, if you have a 1.5GPM shower heard--it will only pass 1.5GPM regardless of the pressure. But having a pump with higher volume might allow you to use the shower and galley at the same time without the water dribbling.
To some extent, hose diameters and fittings and the length of the hose runs will all limit the amount of volume the system will handle, and the capacity of your pressure tank will factor in as well. Bottom line is that you may see some improvement in flow, and there should be no risk as long as you're not going from 2.9GPM up to something like, say, 100GPM.
Some pumping theory that might help. If you have a centrifugal pump (it has an impeller that spins) then the pump is capable of pumping a range of water pressure/volume combinations and each pump will usually have a published "pump curve" which illustrates these characteristics for that particular pump.
The piping system that receives the pumped water will increase its resistance as the pump tries to pump more water - this relationship is known as the "system head curve". With the right knowledge you can actually calculate the system head curve for a specific piping system and then plot this on the same graph as the pump head curve. Where the two graphs cross is known as the duty point and this point is the actual performance of that particular centrifugal pump/piping system combination.
On the other hand you could have a "positive displacement" type pump where the internal "slippage" of a centrifugal pump is almost totally eliminated. This type of pump will pump a more or less constant volume of water under most conditions. If the piping system is inadequate to handle this volume of water, then pressures will rapidly rise until you could get pressure induced breaks unless a pressure switch is installed.
Also unless the 1.5gpm shower head is fitted with a sophisticated pressure control system, the volume of water that it passes WILL vary with the pressure.
Lastly the referenced water system has a pressure tank. You will need to check to see how this is plumbed. If all the water from the pump has to pass through this tank, then nothing you do to change pump performance will have any impact on water pressures at the shower as this tank will control all downstream pressures. If the pumped water can reach the shower without going through this tank, then the selected pump will make a difference.