Good points being made by all - the best IMVHO being that every person needs to experiment with as many configurations as they can to determine what works best for them. I don't think that cold water, a drysuit and heavy underware automatically dictate the ST27s for every diver. I'm 5'10", 160lbs and I sink like a stone. I dive all year round up here, but at the same time I ALWAYS get cold - not a question of if, just when!
So there isn't enough undergarment on the entire planet for me ... but that being said, the ST27s still had me "walking" like Mark said more than swimming. Just horrendous trim, way too much weight in the trim pockets just to try and compensate and with the corresponding back pain after every dive.
Having the weight in the tanks for those that need it definitely makes sense, but there are those for whom the
position of the tanks (by virtue of the overall design) is just as big a part of the equation. I tried out the AL13s after doing my training and a few more dives with the ST27s. I like their bouyancy and trim characteristics, but I just feel more comfortable with a little more gas on the longer and/or deeper dives. So I ordered a set of the AL20s and have 3 dives on them now, as well, and they exhibit similarly better trim characteristics with a few pounds in the trim pockets. I'll alternate now between the AL13s and AL20s depending on the dive plan(s), but my ST27s are now in the same pile with my sets of ST85s collecting dust!
The moral of the story as always being that every diver just has to sort out exactly what works best for them ...
Cheers,