None of the grinding races require skill... Well, the Tokyo one does require a bit more effort for the average player, but that's the only one.
Regardless, we are talking about grinding. I can drive the Tomahawk X while braking on the straights and still be massively faster than anything else. 18 minutes for 825k >> 30-33 minutes for 825k. Grinding is all about getting as many credits within the lowest possible time. It's always going to be tedius, especially in GT7. So if we do have the glitch, just use it... The game and the developers don't offer or intend to give you better ways to grind.
I'm the opposite, I use 7th gear all the time... For both fuel saving and for the car to be under control.
I don't know how in the blue hell you can reach 530kph on the straight and not lose control. After about 470-480, the minimal steering input will unsettle the car. So either you are using a different setup or have godlike car control, and if so, I would really like to see a video of you doing that straight consistently at over 500kph and not lose control. I can maybe do 1 or 2 laps with a ton of luck and reach 500kph and not crash... But usually I just lift and slow down to about 470-480, sometimes even less than that.
Here's my run, I put it on the OP as well: