Yes, but 150 rounds takes a mag change (or several, depending upon how large they are). You haven’t taken that into consideration.
True, but that is beside the point. Cyclic rates on full-auto-capable firearms don't take into account mag changes either.
Based on what I've seen in person (30-round dump in 3 seconds), and on YouTube (30-round and 100-round dumps in 3 or 10 seconds, respectively), if you had a 150-round mag, and a bump-stock, it is entirely possible to dump 150 rounds in 15 seconds.