I thought i'd get all the GT300 Base models and see which was fastest around Suzuka (my favourite track after the Nurburgring). Then i thought i'd see if anyone had done a comparison on here and found your post. Since it's about the very track i just did my test on i thought i'd share my results;
Lexus IS350 '08 - 2:11.539
Autobacs Garaiya '08 - 2:09.425
Subaru Impreza '08 - 2:09.362
Subaru BRZ '12 - 2:10.369
RE Amemiya RX7 '06 - 2:06.034
I drove each car for 5 laps with Traction Control on 1 and ABS on 1, no other aids, using a DS3 controller. No tuning or modifying, each car on Racing Hard tyres and gear ratio's as standard. Times could easily be improved upon but i was aiming for consistancy, often getting my fastest time on lap 3 or 4, and pushing harder on my 5th lap (usually having brief encounters of the grassy kind as a result).
The RX7 is my favourite for somewhat obvious reasons. It handles very well as standard and only gave me the slightest bit of trouble on one corner (the exit of the hairpin) but that was probably my over zealous throttle application. The Impreza is probably the pick of the bunch though, seeing as it's 4WD you can get the power down very early coming out of a corner. If you're a novice driver and using a DS3 this may be the car to choose. The Garaiya was a bit of a suprise for me. Being MR i expected it to be a monster (this is only the 2nd time i've played GT6 since 1.03) but it wasn't. It's surprisingly stable and i'm sure can go quicker than i could make it go. I struggled with its balance, it wanting to oversteer in 2nd gear and sometimes into 3rd, and its general mid-engined traits, but a quick play with the LSD would solve alot of that. The BRZ was fine. Not much more to say really. Handles well, almost to the point of being boring. The Lexus was a little dissapointing for me. It won't be pushed or hurried. If you do it'll suddenly oversteer on the slower and mid-speed corners. It felt kind of numb but was stable enough most of the time.