The Erebus Motorsport driver has been taking part in a limited test session at the Ipswich circuit today ahead of the planned demonstration runs and two days of testing in Tasmania starting late next week.
The Camaro was meant to be joined by the Mustang at QR, however a COVID-19 outbreak at Dick Johnson Racing meant the Ford couldn't be prepared in time.
That left Kostecki as the sole driver to sample the prototype off the back of the chassis changes aimed to improve ergonomics, particularly for taller drivers.
According to Kostecki, his early impression is that the changes have had a positive effect on driver comfort.
"A few of us struggled with few ergonomic things in the car last time we were out here," said Kostecki. "So it's great that the guys and girls at Supercars have worked hard to make sure we're nice and comfortable when we drive, so it can be full attack mode when we're out there.
"I've only done a few laps so far but it's a lot better on the inside, that's for sure.
"It seems that all of the problems have been solved for me so far. I'm pretty happy with inside the car."
Another significant change to the Gen3 prototypes is fitting the same mechanical gear shift system that is used in the current-spec cars.
Supercars had planned to use an Auto Gear Shift and paddle shift system with its next-gen hardware, however feedback from fans and drivers ultimately led to that plan being dumped.
"I was a bit lost last time I was driving the Camaro [with paddles]," said Kostecki.
"It's great that I've got something to grab on to while I'm driving around.
"I like that the stick shift is back. It puts more in to the driver's hands. We call it the 'oh shit handle' when we're going for a pass, so it's great to have it back in there and I'm glad to see Supercars really listened to the fans.
"There's been a few other changes as well within the engine and the drivability. And just having the stick shift in there as well, throws a few spanners in the works."