Sorry, I must have missed your post.

With heater, I would expect about 15 PSI and with heater bypassed, I would expect 11 PSI so there is a large dependency somewhere. I am not sure where it could be. With the pressure differentials you are measuring it doesn't seem like it would be the heater or filter. Removing those does not change the pressure much. But this is probably the cause of all your problems.

Also, the 1.5 HP Whisperflo is going to be a much bigger pump than you have now so I would not recommend that.