There is no public API to manipulate virtual scrolling parameters, except switching it on or off completely -
grid.setVirtualMode(true/false); // default is true
If you want to change the existing behavior you can modify the source code in /source/lib/grid/_virtual.js, line 38-39:
var delta1 = dir < 0 ? 10 : 1;
var delta2 = dir > 0 ? 10 : 1;
These two variables define the number of rows rendered above and below the visible area. By default this is 10 rows in the direction of scrolling and 0 on the opposite side. Both are 0 during the initial rendering and during the horizontal scroll.