I have no answer for your problem yet, but I know for a fact dat Powerpost 2.4 can post at the same speeds as 2.3 and earlier builds and perhaps even at higher speeds due to some improvements.
So, there's something going on...
How many threads are you using to upload?
Are you using SSL on both programs? (or on one, or none at all)
What is your upload and download speed according to your internet provider?
What speeds do you get according to your usenet provider?
Don't forget that 20mb is something different then 20MB
(20mb (megabyte) = 2,5MB (megabit)