I'm not sure that modelling provides any useful analysis. Xero's path is very unique in that it is attacking markets in an upward ascendancy rather than simply flat or linear. It is counter-productive to talk about growth decay rates now when you look at Xero's stages of entry within its various markets.
There's also an expectation with regard to Xero's USA entry that seems to be misunderstood by many posters here.
At the moment, Xero has achieved dominance in NZ. It has won the war here and has taken the market. It is still posting 100%+ growth in Australia and is the leading cloud accounting provider there and, IMHO, will become the leading accounting vendor there. The UK is the engine for the next 1-2 years in terms of bolstering customer numbers. The market there is substantially bigger than Australia's and they are only just now beginning to dig their claws in. They've got brand recognition there and the snowball has well and truly began its roll down the hill... 114% growth and it is likely this will increase for the next FY. Big numbers will start to come from the UK. Along with OZ, this is Xero's turbo button to get it to the half-mill.
When we talk about the US.. they've tripled their numbers from 6000 to 18000 in one single year. I don't believe any of their previous markets (NZ, OZ or UK) posted numbers like that this early in the process. What people have to remember is this is not a consumer play - it's not going to be driven by some viral phenomenon etc and explode with sudden stratospheric growth. It is going to be achieved through simple hard graft by building out their partner channel, fostering that tight community of professionals, and letting them drive the growth. The 200% growth in the US shows that the solid foundations have been put in place and now they can begin to push hard. The US listing could be a big spark to the powder keg if the timing is right and add further fuel. But in terms of customer numbers, the US mightn't start to show a meaningful contribution to the overall Xero picture for another 12-24 months.
As for competition with Inituit... well.. I think it's all a red herring. Xero are carving out their own segment with their own breed of partners who have grown in a new era of technology and are looking to break out from underneath the shackles of software from the last century. Even if Intuit have been a little more forward thinking than MYOB or Sage, they've still exhibited a complete lack of understanding with regard to how to execute or maintain a cloud accounting platform. It's just not in their DNA... it's a whole different beast. Xero doesn't need to win all Intuit's customers, though that would be nice. Xero just needs to plug away like its doing. Already it's clear to see there's a partner channel in the US receptive to what they're doing... in my mind it's inevitable that this will grow and grow and grow, and with the momentum Xero is achieving elsewhere around the world, I'm very confident of their long term success.
On another note, I think the whole fixation on value is also a waste of time. A year ago Chris Keall asked if Xero was worth a cool half-billion. If you asked him that now, he'd probably say absolutely yes.. but also ask yourself, what's changed from a year go? Nothing. Xero's growth has been steady, their product hasn't changed, they've achieved what they've set out to do. While there isn't any direction on profit (and won't be for some time), Xero's market cap is driven purely on the basis of amount of people who want to get in on the Xero story versus the number of people who want out of it. Ask Chris Keall if he thinks Xero is worth a cool 5 billion? Of course he'll say no, as will most, but conjecture about the merit of its market cap ends up leading people in circles.