I don't know all the details of how a difference in voltage translates vs. the resistance of the atty, etc., but I started out with a 510 starter kit and an eGo starter kit, so I know the eGo 3.2V experience. I now use a KR808 variant (Bloog) with cartos. The only real comparison I can make is that the 3.7V of the 808 gives a "warmer" vape, and to me the vape feels a lot more analog-like. I cannot use my 510/eGo stuff now. It's just not as satisfying, by a long shot.
You do not need a syringe to fill the cartos I use. They have a pop-off silicone "drip tip". You can actually drip them (in small amounts), and when you need to fill them you just take off the tip and drip 20-25 drops into the carto, let it sit for about 15 minutes, and it's ready to vape. I have cartos that are on their 8th or 10th filling, and they're like the day I opened them. As soon as one goes a bit nasty, in the trash it goes -- they're only about $2.00 each. I don't even own a syringe.
Bah, looks like more expenditure for me for a true 3.7v battery now...
Warmer vape and more analogue like feel is just what I could do with, yes the ego vape is very cool...
Anyone know where I can get one as cheap as possible (but not a piece of cr%p) that will fit my tank attys and ego xl cartos...?