Ok. Finally did it with 64 bhp, custom transmission, stock tires, nothing else. No suspension mods, no irreversible engine or chassis mods, either.
Transmission tuning is everything here. You want 5th gear at 150 km/h, because at 140, 4th is too short and at 160, 4th is too long... which means you lose speed on some uphill sections and on the loop.
It is possible to win this with a much cheaper build. On Comfort Mediums, I overtook the first place guy at the last corner and blocked him till the finish line... after two sloppy laps gave me back to back 2:17s after I did two 2:16s.
You could probably go slightly cheaper with Sports tires and maybe just 50 hp (with the 2nd stage turbo instead). With Sports Hard and 65 hp, I won the race by the 4th lap (still sloppy). Will see if there's a "cheap" tire-power combo that can win. At significantly lower power levels, no amount of tire will make up for the time lost on the straights.
Going for this build, you still win about 150k credits. Not bad for 5 laps' work.