How about using supercapacitors to convert 5 seconds of 60kW into 15 seconds of 20kW (less losses)?
5 seconds of 60kW = 300kJ. Supercapacitor energy densities are in the range 0.5 to 15 W-hour/kg according to Wikipedia. Say 5 Wh/kg, = 18000 J/kg, so you'd only need a few kg of supercapacitor to make this work. The only price I find is US$2.85 per kJ in 2006, putting the cost at around $1000, probably much less now (but there will also be costs beyond just the supercapacitor.)
You could also make this an option - not much point paying $2000 for this capability if the car is going to be in Singapore.