Those Masscontrol type devices have no control over the pressure. As soon as the pump is started you have max pump pressure, which I am sure is much higher than pressure from a pressure tank. So when you open a faucet you see a lull in pressure, then suddenly get a big gush.
A pressure-operated device, as with a pressure switch will have an upper and lower limit to the pressure like on at 40 and off at 60 PSI. Then adding a CSV set at 50 PSI makes the pressure steady out at 50 PSI for as long as you are using water.
It is much steadier pressure than on at 27.5 PSI and suddenly running at 70 PSI, which is probably what you are seeing with the device you have.
Now if you really need 70 PSI, you would run a CSV set at 70 PSI, and use a 60/80 pressure switch. Your minimum pressure would be 60 PSI, your running pressure would be 70 PSI, and it would only go to 80 PSI to shut off when no water is being used. We could even tighten up those pressure settings to a 10 PSI differential if you like. But it is rarely needed because 20 PSI difference is not very noticeable, especially since it is just on start up.
It is not a pressure tank that causes the big difference in pressure you are seeing. It is that “tankless” device that is causing your problem.