The following is a direct quote from the response that I received from Mary Kaye at PADI T&E concerning the drysuit & buoyancy issue:
"In the PADI Dry Suit Diver Manual, Product 79901 page 30, under the heading BCDs, Weight Systems and Argon Inflation Systems, it states, “ …you’ll learn that it may be more efficient to control your buoyancy while underwater using you suit, not your BCD.” The text goes on to explain that a BCD is always worn with a dry suit primarily for safety in case the dry suit malfunctions, providing the ability to control buoyancy underwater while you abort the dive, and for surface support since fully inflating the suit at the surface is uncomfortable at best. “Therefore, you’ll use your BCD when swimming or waiting at the surface.”
Also in an e-mail from Tom Leaird of YMCA SCUBA:
"First, when diving with a dry suit it always advisable to use a buoyancy compensator in case of catastrophic zipper or other failure. For liability’s sake we would never teach diving without a BC.
Now, where to put the air during a dive is up to the individual but here is how I teach it and is generally acceptable.
At the surface the BC is used for floatation and dumped completely upon descent. As the diver descends, the quantity of air in the suit is compressed and therefore the amount of insulation is reduced. So, it is advisable to maintain a constant volume in the suit, thus maintaining both warmth and buoyancy.
In addition, the amount of weight worn should only compensate for neutral buoyancy on the surface with no air in the BC and a relaxed breath. Over-weighting will cause greater changes in buoyancy during the dive. Of course, positioning of the weight is important for trim and balance.
During ascent from depth many suits exhaust automatically and need no effort on the part of the diver. However, some suits must be manually purged making ascent air dumping more critical. "
Hope this helps...
Ron sends