Little enough to mean that it is not worth the money. The improvements were not something that you would even feel in SOTP performance. You might see a negligible increase on a dyno pull and that's it.
If your dad's Viper is more heavily modfied, then you might see more of an improvement. If you really want to read more on it, go to the Viper Club web site discussion forums and do a search on Mopar Performance ECU or Sean Roe VEC-1.
I would save your money for other mods. What does your dad have on the Viper now? Has he ever attended a high performance driving school? This is probably the best money that can be spent on a Viper (IMO).
Brad