ABS OFF (0) should be no problem in GT6 - it would be pretty much the same as in GT5 (with new physics, their properties, etc., of course) - if the "Racing Brake Pads" were not included.
In my opinion, the "Racing Brake Pads" are the MAIN PROBLEM with the ABS OFF driving in GT6 and it seriously influences behavior of the car and brake-balance setting, especially for the drivers that are driving with the wheel/pedals.
What it seems to me - and I tried more than few cars - is that Racing Brakes are some very simple multiplier of the something I will simply call *brake force*.
For instance, where "normal" brakes have *braking force* of "3", racing pads provide *braking force* of "9".
And there comes a problem, because such multiplied *force* is directly influencing behavior of braking in terms it makes it 3X more sensitive. Where I need to apply a 50% of pedal force on normal pads, I need to apply only 15% - AKA just touch the brakes with tiptoes - with Racing Pads. And like that there is no threshold, almost no travel, nothing.
So, it has nothing to do with *brake balance* because the brake balance setting is just fine and works with the same logic as in GT5 and it works fine (I agree with your formula above Griffith, but I would simply stick to the more basic "calculation" where a value of 1 is always used for rear bias, and front bias is altered with vehicle-weight in mind) - as long as Racing Brake Pads are not fitted on the car.
For example, I am aware of the
thread about "problematic" MR cars, and in my opinion,
the only true problem are the Racing Pads on those cars. Although that thread nominated Cizeta V16 and F40 as *problematic*, they are not problematic at all, I can drive any of them on either Comfort or Sports tires and without ABS (and proper brake-balance settings) and they are acting perfectly. However, once the MR/RR cars with Racing Pads comes to picture (from Huayra to infamous Audi R8LM, to Renault Gordini for instance) everything falls apart because actual *braking force* is too strong because of the Racing Pads and no brake-balance settings can fix it.
Because the actual mass-center of the FR/FF vehicles with Racing Pads is not sensitive as on MR/RR cars, similar difference in behavior is not so exaggerated, but the same problem persists - because actual *braking power* is way too strong - which can be observed by watching the redline on the revmeter that represents braking when driving.
In my personal opinion it is the Racing Brake Pads and way that they work that is greatest current problem of GT6 and they are practically making more than 50% of cars unusable for those that opt to drive without ABS.
Polyphony should drastically change the way Racing Brakes (brake pads) are implemented and instead of multiplying actual *braking power* though current pedal-travel, they should somehow change it in a way to make it more effective in stopping the car by allowing the same amount of travel (braking force) when braking but with better threshold or actual efficiency - but again, not with simple multiplying the *braking power* which creates serious problems for all ABS OFF drivers -
and especially those that drive with wheel and pedals - but with making them more *efficent* though grip-levels of mechanical-elements simulation (suspension or similar).
Until then, ABS OFF braking will be seriously problematic for ALL CARS WITH RACING BRAKE PADS and no brake-balance setting will be able to fix it.
At the same time, ABS OFF in GT6 remains a much better experience than in GT5 for all cars without Racing Brake Pads, because it seems that new physics and suspension model takes into concern actual mechanical properties of the braking-potential of a particular car (thus making ABS OFF driving more right) and it provides much more liviable and believable sensation.
**NOTE: - In my assessment, the *ABS* in GT series is not representative/simulation of the real-life ABS, it just uses that "name". ABS in GT is some kind of permanent braking assist *override* that not only prevents wheel-lock on the buffer-bases (dot doing what real ABS doing in the RL), but also have some invisible traction-control that nullify the suspension-modelling and equalizes unique characteristics of the vehicles resulting with the ability to turn while braking and never lose grip. It is an imposed as simple game-assist with adjustable level of assistance (1-10), it is not "simulation" of the RL ABS effect at all.
**NOTE #2: I am driving with Fanatec wheel and Fanatec CSPV2 pedals with load-cell and oil-damper - and still going through hell because of the Racing Brake Pads. I can't even imagine the horrors of braking with pedals that have no load-cell and especially the plastic pedals such as on DFP/DFGT wheels where sensitivity is probably 5X greater than on my set. Polyphony needs to fix this ASAP, it is the greatest current issue of GT6.
More demanding FM5 - at the absolute limit. Comparable tires would be the step up from the lowest (forget the name). With the stock 458 you have to left foot brake a lot into corners while applying throttle to keep the backend from coming around more than you want (only some, if you can brake straight its not an issue), and then get on the gas to plant the rear. The nice thing about that is there is over and understeer when you want it, from weight transfer, throttle, and steering application (again, I am playing with the Thrustmaster TX on FM5, and CSR-Elite on GT6 - if you are debating over controllers, then please, play NFS Unlimited).
Do you use brake assist with the wheel ? That would be un-sim like 
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