I'm still trying to get the whole plus/negative thing straight in my head.
It's easy. ;-) If you have an empty AL80, it will take 4.4# of lead to make it neutral ('cause empty AL80s are pretty floaty at 4.4# of positive buoyancy). Then an additional 1.5# of lead to make it as negative as an empty PST HP100. So, 5.9# difference in lead required between the two.
Just think of the HP100 as an AL80 that holds more gas and has 6 # of extra metal in it to withstand the extra internal pressure when its full.
When you swap from an AL80 to an HP100, you take 6# of lead off your belt (or wherever) and you should have the same buoyancy when the tank is empty. But, the 100 holds more gas - about 2# more. So, at the START of the dive, instead of being 6# negative (with an AL80), you'll be 8# negative.
Being more negative at the start may feel weird. If you empty your BCD, you will head to the bottom more quickly than with an 80. But, what's important is to still be neutral, at the end, when your tank is almost empty. I dived with a friend who used an HP100 for the first time. She didn't like it because she "felt" too "heavy" at the start of the dive. Being a little more negative at the start meant it took a little more air in her BCD to be neutral. More air in her BCD meant it was slightly harder to control buoyancy at depth (which is the same reason why you always try to avoid being overweighted). The difference is small, but she was sensitive to it. She took lead off to achieve the same feeling that she was used to when starting a dive with an AL80. But, that meant she was too light at the end, with an almost empty tank.
