Why not use the old "use what works" method here. I read your post and you seem to have solved it!
Recently my thruput went to hell. (Testing with PCMag Surfspeed2) I was consistently running close to the theoretical maximum -- usually well above 44Kbps. This was with a cheap V.92 Hiro/Conexant Winmodem.
I am now getting 5-10Kbps. I have tested with another Winmodem (Trendnet/Conexant) and two serial controller modems (Trendnet/Agrere and a new USR/Conexant). All V.92 They are near identical in thruput.
If I put the serial units on my laptop they run full bore.
The two computers are loaded with near identical software -WinXP sp3. Tested using the same cables and phone line.
I have cleaned everything out of the registry involving the COM port and old modems that dosen't belong.
The CPU is not tied up and nothing else is running that I can see.
I can only think there is some conflict using resources I haven't found. Or a corrupted driver for some subsystem like the Intel 82801AA or 82820 bridge chips.