The ACR has factory downforce of 10/20 front/rear if I'm not mistaken.
When you say "factory" RJ I presume you means straight from the dealer in GT5??
If so, no, it doesn't have any downforce, despite, visually having what looks like a rear spoiler you buy from GT Auto for other cars and quite obvious front aero devices. Once a spoiler from GT Auto has been put on then it gets a maximum of 40 aero at the rear, can't remember the front.
Same situation as the Lotus Esprit 350, that comes with a spoiler that looks GT Auto, but there's no aero in 'settings', you want rear aero for the 350 you got to buy a spoiler from GT Auto. Whereas cars like the ZZII doesn't have any visable looking spoilers either front or rear, but as standard comes with as much aero as some RM'd car(s).
Aero / downforce does have an affect, but it's where it's being used that's the key.
We did a test way back in January to see the affects of aero:
Honda HSC @ Cape Ring 530pp sports soft tyres, but no LSD or custom suspension - full engine & chassis rebuild - online - all driving aids off.
Tried hammering it around the ring (of Cape Ring) - back ends let's go.
Added Racing Hards to rear, waitied for them to "warm up" - same result, backk end went bye bye.
Took RH off, went back to Sports Soft and added rear spoiler from GT Auto, aero on max - still spun.
Put the RH's back on (so now it had both max aero and racing hards at rear) - it still spun.
Took spoiler and RH's off, back to sports softs and added custom suspension and LSD, tuned it for rear stability - NO spin, solid as a rock, but yes, you could feel the understeer at the front.
But that wasn't the test (what happens at the front), we were testing what works at the rear and concluded more rear grip and stability could be found via adding (and tuning) suspension and LSD than from upgrading to Racing tyres and rear spoiler on max.
That was back in January though and limied to the Ring @ Cape Ring, been many many updates since then though, so not sure if it's still relevant. But most guys I online race with don't use aero for their 530pp cars (sports soft tyres), me too (other than the Cali as I love the way it looks with the roof up) - the PP saved goes to power / weight reduction instead, that's generally viewed as being more beneficial with the guys we race with, especially on 4WD's that have such good natural stability.
H