Heat enhances nicotine's hit and may vaporize more liquid at once for bigger drags. Heat is a factor of how low the ohms (resistance) of the atomizer or cartomizer is vs the voltage of the battery, Slim ecigs are limited as to how low ohms they can use because their electronics are not built to deal with faster draw of current from the battery (low ohms draws current faster and creates more heat and uses battery charge faster)
So a 3.2v battery with a 2.8ohm atomizer will make less heat than a 3.7v battery with the same atomizer and that will makes less heat than a 5v battery mod with the same atomizer (and the 5v mod might pop the atty)
A 3.7v battery with a 3 ohm atty will make less heat than the same battery with a 2.4 ohm atty and if it's a big 450mh+ battery like a fat model or mod, it could use a 2.0 ohm atty for more heat or 1.7 for more (and some people will go lower than that)
But in general, slim ecigs should stick to 2.5 to 3 ohms attys and cartos
Re what is the difference of creating the same heat with a low resistance atty on 3.7v vs a higher ohm atty on a 5v mod? Some say it is only a slight difference. But if you want the big heat you need big batteries or mods so you can use attys and cartos that are low resistance relative to the voltage they are used on.
In slim ecigs, the highest voltage is 3.7v which is KR808D-1 and some of its cartos are around 2.5 ohms and can get hot on the lips (I use a plastic or delrin drip tip as a mouthpiece)
Most popular tank mods do not use an atty.
Some carto tanks and juice feeding attachments are mixed in the mods here
Juice Feeder / Tank | e-Cigarette Mods by Specs