Sorry my friend, I do not mean to be aurguementative or start world war 3 but you have got this wrong.
I don't use the X1 for endurance races, the faster the car the LESS experience points you get, well on A spec anyway.
I know this because I have done endurance races with different spec cars and I always get MORE points the slower the car I use.
I use the following:
Nurburgring 4hr = R34 Falken tyres racecar
Suzuka 1000kms = HKS Evo 230R
You say you get 709,000 experience points for 8 hrs of actual time, OK fine, but I get 960,000 experience points for 8 hrs of actual time, so how can I be the one who's got it wong??? That's 250,000 more than you???
Also, the OP talks about A spec points, not B Spec points.
Yes, if you are doing B spec and you have an X1010 then this is going to be easiest way to grind, but the OP doesn't have a X2010 and isn't talking about B spec.
If you want to keep grinding Suzuka 1000kms for your 350,000, then do so, it's a free world, but I'll keep grinding Nurburgring 4hr and I'll keep my 480,000 experience points for the same amount of actual time..