I guess next time I brew I'll have sparge a bit faster to get my efficiency back down to the low to mid 70s.
This is another invalid conclusion from the infamous “70% is always good rule”. What is happening when you sparge fact is that your grain bed develops channels. The sparge water will prefer theses areas and can oversparge them. This means you extract excessive tannins in these regions while other regions are still full with sugar. As a result you may actually end up with more tannins in the 70% beer than you have in the 80% beer done with slow lautering.
If you want to bring down your efficiency by reducing the efficiency I suggest using more strike and less sparge water. That limits the amount of sparging you can do. Alternatively you can also stop the slow sparge once you have your 70 % in the kettle and top off with water.
Or, of the beer is excellent at your 80% just accept the fact that you are getting 80%.