Yeh I think I made a mistake when I said foreign extra stout as I am actually trying to get close to the Extra stout 6% brewed under licence by fosters. I my self would prefer the other but it is for my Dad who drinks this fairly often so I offered to brew it for him.
At the risk of sounding like a cracked record, Tony - as with very many other Aussies - has totally confused the 4.3% nitro keg version of Guinness with the 6% FES brewed here and bottled, but not kegged. Oranges and apples.
The use of US-05 would be way off the mark. If you are looking to brew the Aussie 6% version, which is unique to Australia, you should preferably use a lager yeast and ferment fairly cool. A good yeast would be the Wyeast Danish liquid yeast.
When Guinness entered the BUL market in Australia in a big way, Tooheys got the contract but because their ale making plant was restricted - and no doubt fully stretched making Tooheys Old - they signed up on the proviso that they could do the beer as a lager. Guinness agreed, provided Tooheys used the Roast Barley based concentrate shipped from Dublin.
Later the contract was won by CUB and they continued to brew it as a lager. The 6% jobbie, as opposed to the keg swill, is a very decent drop indeed. However I noticed last time I had a few bottles that it's reasonably light in colour and is quite see through at the bottom of the glass if you tilt and look through. I'd tend not to use more than say 300g RB in it (I often use up to a kilo in more robust brews)
On the lager subject, no need to panic - you can start it at around 14 and let it drift up to around 19, not necessary to chill the guts out of it or lager for months.