You can beat the R390 @ Test Course using the Poly001 (assumably the F094/S)... I'm assuming you're talking about the Pro/GT All-Stars...
I used the F687/S with default settings except:
Suspension/Camber: Front = 0.0; Rear = 0.0
Suspension/Toe Angle: Front = 0.0, Rear = 0.0
Gear Ratio/Auto Setting = 43
Stability/Downforce: Front = 0.59; Rear = 0.81
My oil light went on during lap 14 of this race (having given the F687/S an oil change just before starting the series). My F687/S started this race with 2487.8 miles and 914 hp, and ended with 2585.2 miles and 870 hp. Expect similar engine wear and HP dropoff during this race (I may have been a little better off with an Auto Setting of 42).
With no other cars, the F687/S will post lap times around 1.42.000 to 1.45.000. The R390 posts times around 1.37.000 to 1.39.000. Hence... YOU HAVE TO DRAFT!!!
What is crucial is falling into the R390's windbreak once the race starts. I was *BARELY* able to catch it, having started from 6th, but it was not easy (these settings aren't tuned for quick jumps).
But, no matter what, you still have to draft. Doing so will keep you in time with the R390, maintaining anywhere from 235-244 mph (378-393 kmph) around the track. Standalone, the F687/S tops out at around 229 at the end of the straights, around 218-220 in the turns...
For the runs that I've tested (I've ran MANY MANY unsaved times with the F687/S against the R390 and GT-ONE), the R390 pits after 7 laps @ Test Course. So when it does, you will likely have to keep going. Actually, I found that pitting with the R390 was OK, as long as barely ahead, or just a shade behind). At some point (whether you pit with the R390 or not), the R390 will catch up with you again, and you *MUST* fall back into its windbreak until it pits again after lap 14 (do not pit with it). Not doing so means not getting the gold (at least that's what it seemed like)...
I needed a lot of drafting practice (NOT "drifting"

) to win these races, especially with uneven tire wear (it becomes easier around lap 10, when all tires go red). The funny thing is.. the RUF CTR2 probably provides a bigger challenge than the R390 (Pro/Turbo@Test Course)...
Anyway, I found that the key was drafting. Not slipstreaming, which would require drafting, but just simply staying close behind the leader for the laps where they don't pit.
Well, I hope this helped a little. You might want to take the downforce down a little more, to squeeze another mph out, but I found that you'll only be able to go as fast as the car you're drafting anyway...
Let us know how things go! Have a great drive!
Hazzard
PS - One last tip... It helps to save the car's default settings, so that you can switch it back without problem...