If the well field is not reverse return (which requires more pipe), you may have balancing issues in the wells - which is to say much or most of the flow is going through only a few wells. This really inhibites the overall performance of the well field as a heat exchanger. This will result is higher well temps in the summer and colder in the winter, reducing system efficiency on both ends.
Another issue - how far are the wells from each other? 20' is sort of an industry standard, but may not have been used because the layout went from 5 to 11 wells. The closer they are together, the more linear feet of pipe you would need to do the same job.
I would ask about both of the above if you're not sure... Especially since the delta t is only one degree.
This all being said - 88 deg F is within normal operating limits -but if it continues to climb the rest of the summer, I would start to strongly suspect the well field as being deficient as I described. Again, know that as the source temp approaches the upper and lower limits, efficiency of the heat pump suffers.