VinylJunkie
Update
 
Ok so I took the plunge and got RX5. Glad I waited - took advantage of the sale now on at Izotope 
 
I can confirm the RX Connect plug in does work with Sonar although adding it directly to the effects bin doesn't work. (at least didn't for me)
 
To get it to work :-
1. Right click the clip you wish to edit.
2. Select Process Effect>Audio Effects>Restoration>RX Connect
3. In the plug in select Repair>OK
 
This will open the clip in the RX editor.
When you've finished editing click send back on the audio tab. A message will appear asking you to open the RX connect plug in your host.
4. Return to Sonar
5. Repeat steps 1-2 above
6. The plug in will now say Press Process to commit changes. Click ok to replace your original clip with the edited one.
 
TOP TIP
Make sure the interleave on the track is set to mono if the clip is mono. It seems to convert mono clips to stereo if the interleave is stereo.