To reach the Labs features, visit our home page at http://sonic.net/ and select Member Tools in the upper right. Authenticate with your Sonic username and password and then see the Labs area toward the lower portion of the menu. (If you've misplaced your Sonic password, you can recover by following the prompts on the Member Tools login screen.)
To make it easier right now, here's a short-cut direct to the tool:
These are the same settings which Fusion customers have had access to for some time now by dialing 611, option 2.
From a support perspective, our goal is to deliver a stable and reliable standard Fusion connection, so please do not contact technical support about fiddling with your connection profile. You are welcome to play with the setting and compare speeds using the charts, but we can't "fix" a connection where upload priority doesn't perform in whatever way you might hope. Sorry! Labs features are free experimental add-ins to your service, and are offered for your use without the full weight of our technical support commitment. Thank you for your understanding.
As a general guideline, upload priority (Annex M) is generally best for shorter loops, and can as much as double the upload speed. It's just not quite fair, but a member who obtains 20Mbps/1Mbps using the standard configuration might see 18Mbps/2Mbps when upload priority is turned on. But in some cases it will yield little or no gains in upload speed, while download always decreases, sometimes to a great degree. On a longer loop where a member is obtaining 8Mbps/1Mbps, turning on upload priority could yield a result like 4Mbps/1mbps. It's a mixed bag.
It would be interesting if you experiment with it to see your before & after sync speeds down and up, as show in the new Line Profile tool or in the modem's interface at http://gateway.sonic.net/. Post them in reply here if you do this testing! If you want to get real fancy, you could even post the performance charts. Note that the charts only update every five minutes, and show a 24 hour span, so you'll have to leave a setting in place a while and reload the charts after a bit to see the change.