First, what card are you using that is taking 30dBm tx power?
Second, what are the actual signal levels/connection speeds we're talking?
Generally, 5GHz links, you want to get the CCQ up as close to 100% as possible. CCQ (don't remember what it stands for) is essentially telling you how often the radios are getting the right info on the first try. 80% CCQ means you're having to resend about 20% of the data again. CCQ is showing the connection quality, however, just for reference, it is not as big of an issue on B mode, just works differently than G or A.
Just an example, if you get a 54mb link at -74 with 90% or better CCQ, getting the signal to the -60s won't make a noticeable improvement. If CCQ is around 50%, you can set the data rates to only connect at 36 or 48, often boosts CCQ up to 100%, bandwidth and ping would be much more stable, generally faster, than at 54mb with lower CCQ.