You say that the Lambo is up to 700 hp but make no mention of other tuning you might have done.
1. I strongly recommend getting the Racing-Soft tyres which will make a huge difference, like switching from Professional to Standard in GT5 Prologue. The hanldong changes so much that it almost feels like an arcade game when you "wear" the Racing Soft tyres.
2. Also, getting a sports suspension will improve your handling.
3. Get a fully customizable transmission and adjust your max speed to as much as you need for each race. The lower your max speed, the better the acceleration.
4. If you still can't nail it with these options, learn the tracks! Get the driving licenses and do the Nurburgring special events so that you will better know the tracks and how to improve your driving. This is the hardest to do but it will help you the most overall.
The lambo is perfectly fine, you should be able to do it. I did it in the Ferrari 458 Italia upped to 610 hp but the Lambo should do just as good provided you improve your handling as suggested above.