Mckes, the benefit of ra-aus rego is purely that you don't need a ga licence or medical. For a 2 seater it's $220 for the first year and $110 subsequent so certainly not cheaper. Only other advantage I can think of is if you didn't build the RV then under ra-aus you can owner maintain it.
Seems pointless to register an rv7 as a severely weight limited single seater, same goes with Piper Pacer.
Ultralights, there are no back seats in an rv7.
There is no doubt that this particular aircraft would be flying illegally due to the 544 limit on most occasions - happy to be corrected.