Well, before we look at the world record...
Highest Individual Test Scores by Sri Lankans
340 ST Jayasuriya v India at Colombo (RPS), 1st Test, 1997/98 [1374]
267 PA de Silva v New Zealand at Wellington, 1st Test, 1990/91 [1162]
242 DPMD Jayawardene v India at Colombo, Asia-Test, 1998/99 [1445]
230 KC Sangakkara v Pakistan at Lahore, Asia-Test, 2001/02 [1592]
225 RS Mahanama v India at Colombo (RPS), 1st Test, 1997/98 [1374]
That puts Atapattu at #4 with 249. Sangakkara is on 265*, third and heading past de Silva for 2nd and looking at potentially the highest test score in Sri Lankan history.