The experienced pair got their first proper taste of the prototype Gen3 Mustang during yesterday's Queensland Raceway test.
Courtney took part in the morning session before handing over to Slade in the afternoon.
They were joined on track by Triple Eight drivers Shane van Gisbergen and Broc Feeney in the prototype Chevrolet Camaro.
The new cars are designed to produce significantly less downforce in a bid to improve racing, with early testing feedback suggesting the aero balance of the Mustang produces oversteer and the Camaro understeer.
That was certainly the experience of Slade and Courtney, who both referenced the new Mustang's lack of rear grip compared to the current-spec Mustang.
Slade did, however, concede that the high ambient temperatures – nearing 40 degrees Celsius – may have played a role in the lack of grip.
The cars were also fitted with the latest spec super soft Dunlop tyre, which has been geared towards higher degradation.
"I guess there are a lot [of differences],” Slade told the Supercars website when asked to compare the existing and new cars.
“It's not chalk and cheese, but the throttle is different, and the power delivery is a lot different.
“How the engine makes it power, and the torque is a lot different, also in its current form. But I know there’s more work to do. The steering feels quite a bit different.
“The big thing is that it lacks is a massive amount of rear grip.
“On a hot day at Queensland Raceway, it kind of gets worse and worse.
“So, you end up with a big lack of rear grip at the end of the run as to what you had at the start.
“Probably just down to the lack of the downforce on the rear, but the throttle is definitely more aggressive, and the engine is more aggressive.
“If I had my current Supercar here right now and I jumped back in it [after driving the Gen3 car], it would feel like I’m driving a limo.”
The lack of grip caught Courtney out during his stint in the car, the 2010 series champion taking a harmless spin on the exit of Turn 5.
"It's very, very different," he said of the new car.
"The grip level is a lot lower, the aero balance... the car moves around so much more through Turns 1 and 2 here at Ipswich, high-speed corners.
"You're losing the rear whereas in the current cars you're driving with a lot of understeer. It makes it quite lively.
"Also without that rear downforce, on braking there's a lot of rear locking. The car is moving around a lot, which is interesting."
Having been a series regular since 2006, Courtney said the move to an all-new car was energising.
"It's fun to drive something different," he said.
"The cars have changed a lot over the time I've been in the category but it's always been a slow, slight change. Now we have a complete change, everything is different.
"It's fun, it sparks some energy.
"We've got a few dramas still. The power steering still isn't right, it's got a bit of a wave in it and a little bit of vibration, which the guys are working on.
"But that's what part of this testing is all about, to try and get rid of those little issues before we hit the ground running at the start of next season."
