Do I need to re jet my carbs now that there's a sports exhaust on my bike?

Possibly. But most bikes used to be tuned from the factory to run a bit rich, so the jets might be OK. It may just need tuning.
The real problem may be that the viper exhaust is not optimal for the bike. The GPZ was never a highly tuned bike. The engine was mildly tuned for reliability and economy. It has a mild cam and a tame timing curve. Engine parts are all designed to work together... replacing one part will not always result in an improvement without modifying the other parts to work with it.