deadsea - 1:46 pm on Jun 1, 2011 (gmt 0)
I've worked with sites hosted in Canada and the UK that have great penetration into those markets but its only 30% to 50% of their site's traffic respectively. Even with good market penetration, the small market size can limit the amount of traffic available.
For these sites, US market share is less, but US traffic still accounts for another 30% of their traffic. Its a tough call. If you move the site to the US, you are targeting a broader market and have more room for growth. On the other hand, its more competitive and you'd almost certainly be throwing away the traffic from the country where you had been hosting.
If the site is UK hosted and the UK traffic is 5% and US traffic is 80%, the answer is clear: move to the US. For the clients I've had, it certainly isn't that clear.