Hi Pete,
Determining an acceptable level of memory utilization is difficult because it depends greatly on your environment (things like code version, enabled features, traffic levels, etc.)
That being said, disabling threat detection is very likely to decrease your memory utilization (especially in some versions of 8.0, which has a few memory leak bugs).
Here is the configuration guide for threat detection so you can decide if you need it or not:
http://www.cisco.com/en/US/docs/security/asa/asa80/configuration/guide/protect.html#wp1042020
Hope that helps.
-Mike
