Undervolting is definitely worth it to try.

I made few tests with that 3g, it really is not that good network standard what comes to energy efficiency. If the data connection is on but passive (no data moving) battery will run out in appr. 15-18 hours. If I use the phone as a bluetooth modem and download at maximum rate (44-48 kt/s) all the time batt wont last any longer than 4-5 hours

At least it seems possible for you to improve from that 5-7 hours.