We didn't spot the 15cm - we played 20cm (same range as for shotgun).
Th player aimed short on the assumption that it would scatter (never a good start!) and it did - back towards him. But we did spot the 1/2 distance thrown rule but it was still close!
We were thinking of changing it to hit on 1-4 on a d6 with an 8 scatter (N,S,E,W, NW, SE, SW and NE) for 1-2 cm (elite), 1-4 (standard) and 1-6 (green) based on general accuracy with (say) throwing a tennis ball. Unless you are completely crap at sports, you will generally get within a few feet of a target you're throwing a ball at - so why when throwing a grenade would you miss by many yards?
But we'd have to playtest to see if it doesn't unbalance the game.