Cisco Firewall :: ASA 8.4(3) Timeout Configuration

I would like to know something with more accuration about idle timeout configuration. In particular why is impossible to set "half-closed connections"  to a value lower than 5 minutes neither through a policy-map? In my  particular scenario, my asa is used to nat mobile phones traffic, it  should be advisable to use less than 5 minutes
 
In my configuration I've set the timers as follows:
 
.
timeout xlate 0:15:00
timeout pat-xlate 0:00:30
timeout conn 0:14:00 half-closed 0:05:00 udp 0:02:00 icmp 0:00:02

[Code].....