I manually altered Ultimate Traffic's FS9 'traffic file' so that it would work in FSX, this is largely just a case of copying and pasting bits from FS9 to FSX. seems to work okay, so I've never bothered with any FSX-specific traffic add-ons.
Here is me buzzing some AI traffic in FSX in the Me262, traffic generated by the FS9 traffic bgl pasted into FSX.
Chock