Yes Nev. As you said, during training in an RA Aus aeroplane one is mostly flying with their instructor hence the students are already used to the passenger's weight. In fact I find it much easier to handle the ultralight with both seats occupied as it makes it more stable in windy conditions due to the added weight; makes the ride less bumpy and makes it much easier to land in a shorter distance and slow down the aeroplane quicker as compared to doing solos. A Jab LSA tends to float down the runway indefinitely until it finally runs out of air (-speed) and settles down on the grass strip /tarmac.
One strategy taught by my instructor is that if you do not touch down by the middle of the runway, it is best to go around. [Rather go around than make a spectacle of yourself, and there is no need to say why you decided to go around if you are really that ashamed!].
I think a similar strategy should apply to all take-offs as well. If you are not airborne by a pre-determined point, somewhere on the middle third of the runway, then abort take-off, especially on a hot day with full load / passenger.