I wish it was the air lock overloading.  A different sprocket on that would be a simple and likely effective fix.:)

More horspower will not make much difference on this pump unless the extra power is used to spin it faster.  It might make a difference if it was a bigger pump.  The motor draws about 30 amps if I remember correctly.  300 bu/hr will plug it on runs of greater than 200 effective feet.  250/hr works at 200 effective feetor less.  15 hp ought to be able to do a lot more than that.  Another run of about 300 feet will knock it down to about 200 bu/hr, depending on test weight.