Hey HeatCker!
Yep... I agree with rwkosh... The Thermalution - and for that matter, heated underwear ('cause it's not really an "undergarment") of any brand name will help you trade bulk and lead for a thinner, simpler system.
In the temperatures you list, a diver might choose to dive either a 200g undergarment or a 400g undergarment, depending on how cold he or she tends to get... Or how much activity is planned on the dive. I dress out warmer, for example, on dives that include decompression stops or long, relaxing immersions. For shorter dives or dives with a lot of current where I will be working hard, I tend to choose lighter weight, less bulky undergarments.
If you add a Thermalution at those temperatures, you'll definitely ditch the 400g and might even go lighter than a 200g and into a Polartech. That is... It'll add about 15-20* to your perceived water temperature.
...And yes, even in a drysuit - the heaters still heat your core (blood) if there's no water layer between your skin and the suit.