Saabkid is right to some degree, 7's are good to about 350bhp or
so. The catch is you can have a tune where the 7's are fine or you can have a tune where 7's are running too hot. It all depends on the a/f, the boost level, etc.
The best way to know is buy a new set of 7's, put them in you're car, do a few wide open pulls and then pull over and take one out and read the plug, the heat range is found on the ground strap and should be right in the middle of the ground strap. If its close to the base ring where its welded on, its reading hot, if its over the electroid, its reading cold.
Do a google search for reading plugs, there is tons of information on this out there and it does not just apply to saabs, reading plugs has been around for a long time and any good drag racer will be reading his plugs after each run and determining if the a/f is good, detonation is occuring, heat range is correct. You can get a lot of information from the plugs without ever looking at one single gage.
You could bring me you're car and I could tell you if its running lean, rich, detonating, has coolant leaking through the head-gasket, etc.
The best thing to do is read up on how to read plugs, buy a new set and then do some pulls and read you're plugs.
That all being said, the 7's are generally fine to about 350bhp, but if its a stage 4, I would say 8's are needed as most stage 4's push the stock turbo to the very limits and that is when the heat gets over the edge and the 8's would be a little safer.
Now a stage 5 with larger turbo that is not being pushed to the absolute limit might still be ok on 7's. The only way to know is to read you're plugs
John