Push processing HP5

I have been developing my own B&W film for about six months now and have been puzzled by a recent result using Ilfosol S @ 1:9. I had exposed a roll of HP5+ at 1600 ISO but had developed it at 400 ISO @20C, (9 min, i think , from memory). The negatives were fine, with nice tonal range and good contrast. XP2 has the ability to have different frames rated at different speeds, because of HP5's latitude can i conclude the same thing? I am currently reading Gene Nocon'c Photographic Printing where he advocates developing at the manufacturers speed regardless of how film was exposed. Does anyone have any comments? Many thanks in advance Andrew

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