Mike_D - Your setup would be a lot lighter with 5s Lipo (as opposed to 16 GP3300s). The only problem is that there is no Balance Pro 3s pack. With 5s 3200, you will not gain more capacity. The Kokam 3200 cells only output 3000-3100mah of actual usable capacity (one thing that I don't like about them). BUT, since they are lighter weight than NiMh cells, your truck won't pull as many amps, which will seem like you are being given longer runtime. But I don't think you would get more runtime than what your GP3300s offer.
Just to add a few things to Batfish's post:
-You can actually mix batteries of different capacities, but this cannot be done with series. Let's say you have a 3s 3000mah and a 3s 2000mah. You can put these two together to make a 3s 5000mah, but you can't put them in series. If anyone wants to know the reasoning behind this, I will gladly share it.
-The safety of Lipos are getting better all the time. But about 80-90% of all the fires and "puffing" of cells (like bloating, from internal gases) is from those that do no maintain/care for their batteries properly. The 3v/cell cutoff, and making sure that a pack IS balanced will decrease the safety issues. Balancing a pack is where all of the cell voltages are as close to equal when fully charged to 4.2v/cell. If one cell in a pack is .1v out of balance, this can dramatically decrease the life of the pack, as a cell like this could charge up to 4.3v, which is above the safe limit of 4.25v. Unlike NiMh cells, Lipos can't be balanced by slow charging.
In regards to what Seth is talking about, there is a website with a forum that is dedicated to discussion about batteries, and Lipos take up about 80% of that. The website is
http://www.rcgroups.com/forums/forumdisplay.php?f=129 A lot can be learned there, and you can stay up-to-date on what is going on with large battery companies (like Thunder Power) because there are representatives that post on there from those companies. Also be sure to look at the threads that are stickied, as they display a lot of good info.