Based on the Golovkin fight, then I can't imagine that Lemieux stops Brook and Brook is obviously a much more skilled fighter. Brook certainly wouldn't lose to Alcine and Rubio.
Same as the GGG fight, Brook does ok but the power and size is too much. Brook is a Welterweight, a big Welterweight, but a Welterweight. Lemiuex in 6.
Lemieux destroys Brook worst than Golovkin, Golovkin isn't just powerful he is highly skilled with an olympic background Lemieux is a killer punching street fighter
Lemieux is slow enough that Brook could do what he did against against Porter; work off the back foot and clinch when needed. I don't see Brook hurting Lemieux but he might frustrate him. Kell crumbled pretty fast against GG but I don't think Lemieux lands enough clean shots to wear him down down significantly. Brook by decision.
If we're talking former GGG opponents, I'd take Murray over Brook.
Brook would stop Lemieux, he would also stop Canelo. He is a natural MW, the second best fighter in boxing right now but there are levels to this and Gennady is the greatest ever.