I know the question was directed to RR, but this is my take.
Holloway is now the form lock for the Tahs. Simmons plays a different role, I know, but Holloway has been much more valuable to the Tahs go forward than Rob, and he is also very good at the lineout.
In Wallabies' contention, then, I'd have Coleman, Rodda and Arnold as the three absolute standouts, with Tui, Philip and Holloway next in line. Simmons would be vying with the likes of Carter and Enever, a fair way down the list. Then at the bottom would be Hanigan.
The arguments in favour of Simmons are his lineout calling ability and his reputed scrummaging. As I've commented elsewhere, I believe all of the current crop of Wallaby forwards know the calls inside out and backwards. The calling is just a matter of judgement and that can be (and has been learnt by Coleman for example) learnt in fairly quick time. I don't think any one of the other locking contenders loses anything to Simmons in the scrum. Arnold, Tui, Rodda are members of the strongest scrums in the country. They don't get to that level with a lock who is poor in the scrums.