According to this Wikipedia entry, immigration from Sri Lanka to the US did not happen to any significant degree until the mid-'70's. Even now the number of Sri Lankans/Sri Lankan-Americans in the US is relatively small I think (i am not sure what it is, probably not even half a million, probably not even a quarter of a million). Out of that small group only a few have pursued careers in the arts & entertainment, let alone filmmaking, as far as I know.
Recently I finished a one week run & the 20th screening of my second fiction feature, Date Number One (2006), in the US (screened so far in NYC, Seattle, DC, & a couple of MD cities). DNO might be the second US fiction feature made by a director from a Sri Lankan background (and in my case, actually from Sri Lanka, I immigrated in the mid-'80's, as a young teen), after Wild Diner. Soon/hopefully in early August the DVD of DNO will be available (and at some point in the near future Wild Diner & other earlier works will be available on DVD also - not sure if anyone really wants to see that stuff, but it'll be available :). Anyway, I have not heard of any other US based filmmakers with a Sri Lankan background making and or releasing fiction features. Ever. Never in the relatively brief (compared to the 2,000+ years old Sri Lanka) history of the US.
So, it is quite possible that I am number 1 or 2, or at the worst in the top 5 for "filmmakers from a Sri Lankan background who made & released/screened a fiction feature in America" :) A little obscure factoid for us to break out in like maybe 50 years or so :) Maybe sooner.
Of course the "1st widely/Hollywood level released fiction feature by a US filmmaker/director from a Sri Lankan background" title is not claimed yet. I guess something for me to work towards, possibly :)