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Not even that much. The 89 Si was the D16A6 with a whole 108 non-vetc Hp. The D15B ("D15B2" I am assuming) had/has 92Hp. It would be an upgrade, but not enough for the trouble/expense of a head swap.
Oh, my bad. I was thinking of the D15B1. I didn't notice that he said DX... But please, if you're thinking of doing something as strange as a Y8 head on a B2 block, just... don't do it.