My understanding is that, while the Protecta harnesses are sold as being safer than just the adult seatbelt, tests have shown that they can actually cause injuries if fitted incorrectly and it's pretty easy to stuff it up. Kidsafe Victoria actually post a warning on them in their child restraint factsheet here There's some detailed research on this around somewhere which I can try to dig out if you want it.

If your child has outgrown the 5 point harness by weight then they can go into the booster mode, however has PP mentioned, they are actually tested up to 22kg so there's some leeway. I'm going to have the same issue with DD2 who has just turned 3 and is 17.5kg now. I'll be leaving her in the 5point harness up to 20kg unless she outgrows the straps height-wise (still a few cm to go so it's a while yet!) On the other hand, DD1 has only gone out of her 5 point harness recently at age 5 after only just hitting 18kg I bought her the Infa Vario booster with the adult seatbelt due to it's high safety rating. I'll probably get DD2 one of these when I have to convert her too as we have the basic Maxirider which doesn't rate too highly in booster mode.