According to Computronix' Temp Guide (latest revision, 2.8.08), SpeedFan, even 4.33, is correct for MO stepping chips, which have a Tjunction max of 85 degrees, not 100 degrees (this was a mistake in the previously posted version of the Temp Guide). I have an e4600 (also MO stepping), and SpeedFan, both 4.33 and the Beta, report the same temps, which are consistent with CoreTemp and other temperature monitoring apps.
Edit. In other words, no 15 degree offset is necessary.