You're OK on the temp. The overflow jug in conjunction w a coolant recovery cap stops the majority of coolant loss (some still evaps as steam out out the jug as it has to be open for it to fill) and after it cycles a few times and gets most of the air out of the system w the coolant right up to the rad cap seal that can let it run a slight bit cooler and lessens rust formation as it needs air to form. To ans your Q if you are overheating (or not) it will be independant of whether you have an overflow system or not. Is that the temp around town or out on the hiway?