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Right around 20-25rwhp. More if they are ported and more if you mill them to raise compression.
IMHO 20 - 25 RWHP is wishful thinking. I believe 10-15 is a more accurate number. Ported LS6 heads (243 castings) will get you 25-30 or even more depending on your cam. They can be had for under a grand.