I did a quick test on Suzuka using those S2000, best out of three laps each car.
Every car was engine rebuilt, oil was changed, and confort soft tires were fitted. I drove using no aids and H pattern manual shift plus clutch.
The regular S2000, fully stock offers good amount of grip, slides a little bit, but nothing uncontrollable, it has short gear ratio so it reaches 6th gear on the straights with ease. Nothing more to say, it's your regular S2000.
Laptime was 2:34.9
Mugen S2000 offers good amount of grip while cornering, but the back feels a bit heavy at times entering the corner, it tends to understeer under braking, but still predictable and enjoyable to ride, it somehow has a bit wider gearbox, feels a tad sluggish in 4th and 5th gear, but overall it offers a great balance.
Laptime was 2:32.4
Amuse S2000 Street Version, it's the fastest of the lightly tuned S2000, though it's a tricky ride at times, it dives in very well into the apex, but the back end feels way too stiff, making it bumpy and very prone to slide on power, it has great acceleration and great top end speed, it has good brakes, very balanced, so even under heavy breaking the car still responds well. Although its inherently fast, you'll find yourself countersteering quite a lot coming out of the corners.
Laptime was 2:30.8
Then there are other two that are in another league:
Opera Performance S2000, its very light, very fast, it has the best acceleration of them all, great traction coming out of the corners, tents to slide a bit sometimes maybe due to the lack of weight on the back, but it just blaze through the corners, right goes right, left goes left instantly. I gave up on testing this one at the very first lap and it still clocked 2:23.6
Titan R1, it's heavier than the Opera, yet it's still as fast, maybe has the best driveability of them all, it corners like a dream in high speed corners due to it's massive wing, and you always feel on crontrol of every situation. And the sound... Although not the fastest, it's my favorite ride of them all. Gave up on testing this on the first lap as it's way faster than the lightly tuned cars and it still clicked 2:25.8 with a very bad lap.
That's my view of the stock cars with stock settings.