I must have been lucky in that Viper race...
First try was slightly tuned 2013 GTS with sport soft tires and had not the slightest chance to even follow the slowest car and quit...
So I tuned that car with most available parts except the supercharger(no swap though...), used all 5 weight reductions and put on RMs and all aero parts (and made a livery for it...cannot race a car I put so much tuning work into without doing so

)...it come out with 780PP(619kw/1151kg)
First lap with that car was a total mess, I overshot pretty much every turn or corner. not getting one single braking point right, so I quit again after one lap...went back to the tuning shop and equipped it with carbon brakes(one thing I didn't in my previous tuning stint)
On the next try I was driving really safe in my first lap braking really early(was still on the last spot after that lap. but less than a second behind the next car), and went a bit quicker and more daring with every following lap, gaining position after position and taking the lead by the end of lap 6 and kept the opponents at bay in the last lap(altough any greater mistake would have cost me 3 or 4 positions..) and did win that race...
That car is a beast that needs gentle treatment and likes to coast through the corners, high gear/low rev is the secret there...on the two long straights it goes off like a rocket and leaves everything behind...but it hates full braking afterwards, instead lift the gas pedal earlier than you normally would, coast, and break as gentle as possible(and again, turn in on higher gear than you normally would and keep the rev as low as you can)...us the uphill part of the tunnel on the back straight as well to slow you down...
(that all was on medium difficulty)
And one more thing; never ever hit full throttle before the nose of that car faces exactly in the direction you really wanna go...
Worked well for me, maybe it is some help to you...