The poor default brake balance (really, it makes no sense) for all cars in GT5 doesn't help in driving with the ABS off.
Under full braking usually at least 75-80% of the car's weight goes on front wheels. The brake balance should reflect that.
I usually use 9:3 or 8:3 as a good starting point. This allows to lock front wheels at just over half the total brake range with comfort soft or sports hard tires on a flat road, without downforce.
A more practical way to set up the brake balance is to set up the front brakes strenght to the maximum you can handle without immediately locking wheels (make sure you still are able to lock wheels, to have reserve power in case of added front load due to elevation differences or aerodynamics). Then set up the rear strength to the maximum you can handle without having excessive oversteer when cornering or worse, locking rear tires. Rear brake strenght will be lower than the front.
You're correct that the weight shift means more front braking is needed (see my earlier comments re bake [or even brake] disc sizes). However, my feel of GT5 is that the "natural" brake balance of each individual car, with a bias to the front, is already worked into the game at 5/5.
I regularly drive on GT5 with the ABS off, and a 5/5 brake balance (or otherwise, the same value for both front and rear brake strenght) means a 50%-50% brake distribution.
This will cause rear tires to lock first when braking, and the car to spin hopelessely when approaching corners.
Please note that with the ABS enabled brake distribution values almost completely lose their meaning.
I was at Laguna Seca and found that too often I was locking the rears, and tinkered with the brake bias a lot until I was a bit more comfortable (I think 1:0), I felt as though I would be happier with the fronts locking up more than the rears, it felt more manageable.
Keep in mind that if the brake strength on the front axle is too low, you won't be able to apply the maximum braking potential. You have to make sure that you are able to lock front tires. A brake distribution of 1:0 is too low, try practicing with higher values until you are able to apply the maximum braking force at something between 50% and 100% of the total brake input (pedal or DS3 trigger).
Don't shift the balance to much from 5/5 as you will be asking one axle or the other to do too much of the braking, resulting in inefficient braking. If you are having to set balance too much away from 5/5 with racing cars, I suggest your suspension settings are not working well for braking and the tyres you are using.
In real life it's the front tires that do most of the braking anyway. You have to make sure that they lock up before rear ones.
As I've written, with a 5/5 brake distribution rear tires will lock first in GT5.
I have a wheel (DFP), and I adjusted the brakes to f=1 / r=9. Is it better to adjust vice versa? Plus I always try to brake before the turn.
You drive with the ABS enabled, don't you? With it, the brake distribution appears to lose its meaning. If you disable it you will spin immediately when braking fully.
Your braking distances will also be much longer than normal if you manage to not lock rear tires.
What sort of controller do you use?
A properly set up brake balance is manageable on a DS3 too.
I use ABS=0 on both that and a DFGT wheel.