Should probably be more than 15% in order to attract sufficient attention, yeah. But although that's not too good for me, personally - give the bonus only to those whose salaries (-bonus) are above the salary floor, cause otherwise... well you very well know why. ;)
BBB: 2 (S37 S38); Top tier: 7 (S35 S36 S37 S38 S39 S41 S63); Cup: 9 (S25 S35 S36 S37 S38 S39 S40 S41 S61)