Have you considered a Limiter instead of compressor? I don't do much slap so forgive my ignorance, but from what I gather it's the "pop" part of slap that makes the peaky signals that a lot of people want compressed. But a compressor sort of affects the whole signal, whereas a limiter selectively "squashes" only those peaky signals (in this case, mostly highs as I understand it).
Again, hardly my area of expertise and I could be wrong but it might be something to at least look into.
Again, hardly my area of expertise and I could be wrong but it might be something to at least look into.