Besides the increase in filter size, another reason for the drop in pressure is that the plumbing on the suction side cannot support high flow rates. The Whisperflow should actually be a stronger pump than what you had before so the flow rates should go up and head loss increase as well (read PSI).

However, since you also changed the filter, the head loss on the return side probably went down thus the lower PSI. Even so, I would have expected with a stronger pump for the PSI to go up or at least stay the same. If you have an abnormally high head loss on the suction side it would also show up as low PSI. The only way to test this is use a vacuum guage on the suction side of the pump. If it is much higher than 10 in-mg, I would say you may have a problem on the suction side.

A few other indicators:

What size plumbing do you have? Suction side of 1.5" would be a problem.

Is your pump unusually loud? Pumps get loud when you have high head loss on the suction side but low head loss on return side.

If you can restrict the flow on the return side, does the pump get quieter?

If you answered yes to the last two, you may have an issue with the suction side plumbing.