At Grandmastery you get: +3 to hit, +5 to damage, -3 to speed factor and an extra attack. It's less than Baldurdash fix, which gave you a full extra attack for grandmastery alone cumulative with the half attack from specialization. But it's also more than vanilla BG2 (not sure about original BG1, as I don't think you could reach GM in it).
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pretty grand in my book.