Placing the start lever to idle at 15% N2 makes the light off coincide with max motoring (about 23% around 10 secs later). The engine thus has a continuous acceleration throughout the start.
I realise this doesn't actually answer the Q, but it suggests that GE, being the sensible practical types they are, introduced the 15% figure for this reason and not any technical limitation. Just my 2 cents worth.