I think they would become stronger. Although originally I would have said that the slayer would lose her strength once dead, but as we know Buffy dies a few times and retains her strength, and as per season 9 we know that the slayer power comes from within the body. Therefore if a slayer died and became a vampire it would most definitely be a super vampire, although whether slayer strength + vamp strength would be the case is arguable, I think it would be more likely that one would cancel the other out, so either they would have the full strength of a vamp or of a slayer. As we know slayer strength comes from a demon, and so does vampire strength, so I don't know if two demon essences could coexiist in one body.
Also if a slayer turned into a vampire had the strength of both, then surely she would be almost unkillable, and since we haven't seen such a vampire so far, it could be said they may not exist, rather than no slayer has ever been turned, which is unlikely.
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Another question would be can a slayer be turned, she is in some way demonesque, if not physically... could such a creature be turned?