You can go hot sparge directly into a 155F mash, but you can't go hot into an already "mashed out" mash. You'll equilize somewhere well above 170F.
I think cooling down your sparge would only take a gallon of cold water. You can also compromise by heating the HLT to 175 and let the HERMs intially bring the mash up to 160ish just before first runnings. The 175 sparge will get you all the way up to the 170 limit.
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