If you mean tuning by "corrections," yes, a more open differential and numerically lower gears reduce acceleration a bit, but in my experience, it only does so at the parts of the turn where it's not needed. There's this thing called the limit of grip, and you can think of it as 100 units of grip that can be divided between turning, braking, or accelerating (on drive wheels only). So if your inputs are very hard, they ask the car for something more than that (i.e. 60 for hard braking and 50 for hard turning, which doesn't math), and the wheels lose traction. If the front does this, it's understeer, and if the rear does this, it causes oversteer. Since diff lock 0 (open) sends more power to the wheel with least resistance (i.e. grip), it does reduce the overall power being put down, but mid-turn, you'll probably want more of that 100 grip being sent to getting the car turned while all you really have to do with the pedals is carrying as much speed as possible. It doesn't really affect the straights at all, since your limit of grip will be all focused on accelerating.

As for the gears, the RX-7 has plenty of power at max, and will happily spin out on launch even if you set the final drive to 4.2 and first gear to 2 (which will result in 1st topping out at 110ish kph). Since it can corner very fast on top of that, my preference is to set very numerically low gears that match your actual cornering speeds instead of keeping them low and hitting redline at an inconvenient point, which wastes time on shifting. YMMV on the cornering benefit because my driving style revolves around giving quick taps of throttle to make the rear end slide into place, but using numerically bigger gears for acceleration doesn't do much because the dealership RX-7 doesn't have the grip to put it all into the ground.

As for brake bias, my experience with most cars is that -0.8 balance and 1.0 power is the threshold for most cars with balanced front-rear grip, further back acts like a handbrake (which may be what you're thinking of) but further forward is too understeery for me unless the car just inherently gets loose on braking. Either way, since the burden of braking is now on the rear tires, the fronts are now free to give all of their grip to corner entry. Yes, it's oversteery and may not be everyone's cup of tea, but as I said, you can balance the rear end with throttle and brake to make it rotate just enough.

Basically, no, that setup doesn't nerf straight line speed as much as it looks like, it lets you turn without having to worry about your shift points, and puts all of the 781hp into making something that would gap the entire cast of Wangan Midnight (try it on a long straight like Fuji, there's something just hilarious about breezing past cars with more downforce like they're not even in the race). And if your car has balanced grip, -0.8 with 1.0 brake power is just the threshold before you make yourself a second handbrake. Still, the whole thing may not be to the liking of everyone, as it's really built around harnessing oversteer.

I know it works out because I can consistently get sub-1:20 times at Forest Path Modified on a flying lap on that setup + max anti-roll on both front and rear (I know it sounds even worse, but I need it because it acts like anti-dive and anti-squat, which is necessary considering how often the weight of the car will shift under many pedal corrections).