I've not tried the minim 450 but PHD temperature ratings are pretty conservative. I'm good in my minim300 down to 5 degrees and freezing if I wear thermals. I'm a cold sleeper though.
The zip less design is great unless you are liable to to be using the bag in warmer conditions. I find sticking my arms out is the best way to avoid overheating in that scenario. I reckon the bag squishes down smaller with out the zip too..