Hmm. The '95 manual (section 22) says to evacuate to 27 in Hg or better. Anything less, per the manual, indicates a leak.
I've only evacuated and recharged an a/c system a couple of times, and never on my NSX. But water molecules and air are the big enemies of efficiency in an a/c system. Some shops like to leave the vacuum on a minimum amount of time because time is money. Someone in the business once told me to leave the vacuum on for up to 3 hours to really boil off all the water molecules. So, I did that and things worked well. But, I of course didn't repeat the process and leave the vacuum on for only an hour and compare results.
Let's see what someone with more a/c experience has to say.