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The underhood insulation is there for a sound deterrent. Any heat retention is incidental. If it was important to keep it warm under the hood, they would seal it up just like the passenger compartment. Rubber seam sealed and all.
A much better way to retain heat would be to pop the hood and put said blanket on top of the motor, avoiding any hot exhaust parts of course (don't want to burn the car down).
To answer the OP question - waste of time to try to add additional hood insulation.
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