That's right but this was mentioned many times - card from 5xxx serie and newer does not have problems with halved speed with interlaced resolutions.
Just some info for people having this issue and looking to upgrade to 5xxx.
I do have 5450 Cedar and Caicos having issues with half speed in interlaced mode. Even low end 6450 and 7450 can have this issue.
[AMD/ATI] Cedar [Radeon HD 5000/6000/7350/8350 Series], ATOM BIOS: C09302 -> half speed issue
[AMD/ATI] Cedar [Radeon HD 5000/6000/7350/8350 Series], ATOM BIOS: 13 -> working perfectly
[AMD/ATI] Caicos [Radeon HD 6450/7450/8450 / R5 230 OEM] , ATOM BIOS: GV -> half speed issue
It really depends on the bios version but version number is a bit cryptic.
No workaround for Linux users at the moment. Setting frame_delay or positive sync does not help.