Removing all of the cats from the OEM system can net around a 30kwatw gain with a good tune, let me rephrase that, with a good tuner.
OEM tunes here on 98 are typically between 110-115kwatw on a healthy motor.
I wouldn't waste my money on a cat back with a NA tune.
If your in Sydney I've got a decatted 2017 OEM header and frontpipe you can have for a 10 pack of Double Jacks.
Then use the money saved by not getting a catback to get a good tuner to tweek the tune with Ecutek.
All for the cost of the catback.
I use and recommend Pulse Racing in Menai.
https://pulseracing.com.au/toyota-86...ance-packages/