This would mean 450,000 MR would transfer to 300,000 Marriot rewards which would in turn transfer to 100,000 airline miles. Add on a 25,000 bonus (5,000 for every 60,000 MR) and it's still a rate of 3.6:1.
Surely you'd just transfer straight from Amex at 3:1 unless it was a program not covered...