Pleasure would be good because it results in paperclips, not vice versa. If you reverse the electrodes so that they stimulate the pain centre when they find paperclips, and the pleasure centre when there are no paperclips, this being would start instrumentally value pain more than pleasure, because that's what results in more paperclips.

Minor correction: The mere post-factual correlation of pain to paperclips does not imply that more paperclips can be produced by causing more pain. You're talking about the scenario where each 1,000,000 screams produces 1 paperclip, in which case obviously pain has some value.