no matter how big your coil is, the voltage required to jump the gap will be always be the minimum necessary. Be wary of uprated coils that advertise 1000's of gigavolts. What you need is for the spark to be longer in duration to get a better burn. The coil supplies a finite amount of energy, if you have a wider spark gap you will have a shorter duration and vice-versa and its the duration that counts. Better to have two seperate smaller ones that charge fast than one big slow one. The only other way to upgrade is to minimise the ignition losses, ie use low loss ht leads, have a good distributer with small gaps etc.
If you suffer from spark blow out, try turning your plugs so that the gap faces away from the inlet. Wont make much difference in a 16v but it marginally sheilds the spark.