You can't avoid the "wait". The only solution would be to have Sonar scan the file for peaks each time it is changed. Sonar would seriously bog down your hard drive with all the file scanning if this method were implemented. What we have now is the best trade-off IMO.
As far as plugins go, they only can process what the host app is sending them. So there is no solution on that end either.
Finding a peak requires scanning the entire wave file each time a destructive edit takes place. This wouldn't be a problem with small clips, but may become a serious PITA for those who use bigger clips.
Perhaps these are options that could be added to Sonar: A) Update peaks after destructive edits.
B) "Scan for peaks" on right click menu.