have you got Sr3 installed? as i stuck with Sr2 'cause everything seemed to work so i didn't see the point of fixing what ain't broke! so maybe if you have Sr3 that may be a bug/side-effect of it. ( i have a memory of hearing something along those lines, a while ago)

but i don't really ever render over backgrounds anymore, i always export my render out as a .jpg and .png, the .jpg is incase i lose some hair detail with the .png, and the .png is what i place over my background, that way i get to do more overlay effects and such in photoshop.

have you tried to double the render dimensions, like 1700 x 900 at 300dpi, then adjust it in your image editor, to the size you want?