bigger doses don't help, more frequent dosing does.
that's why entacapone, mao-b inhibitors are added in hopes extending how long an adequate blood concentration of l-dopa can be maintained. getting enough l-dopa into the blood is usually not the problem, that's the easy part. the hard part is keeping it constant in the advanced patient - yopd'ers are still producing their own l-dopa and protecting it from being destroyed in the brain, i.e. they still have normal l-dopa production so the oral l-dopa you take is just a "top up, which is not possible 24hrs a day with current oral medication imho which is why l-dopa pumps are now in play, and i think they actually use more l-dopa than when a patient uses pills, go figure

in the book "UNDERSTANING PARKINON'S DISEASE" by richard b.rosenbaum, pg 99, where he is describing the early research where patients were given l-dopa, some OVER 10 GRAMS - carbidopa hadn't been invented yet(?) - and this was a surprise that such a high dose was needed to be effective when it was estimated that the brain only needed 7.5mg of l-dopa assuming it could diffuse to the parts of the brain where it is needed.

so what explains the "wall" where increasing the dose doesn't help? must be that the there just aren't enough brain cells left that produce the enzyme that converts l-dopa to dopamine to outpace the enzymes that break down l-dopa. so all that extra l-dopa in the blood doesn't help, once it gets into the brain it has to diffuse where it's needed so the limiting factor much diffuses to where it is needed and how fast it can be converted to l-dopa, all the while being broken down by mao-b? i'm sure it's much more complicated than that. i picture it like you have a huge crowd waiting to get into a concert hall with set number of entrances and set number of chairs, once you saturate the entrance and chairs, doesn't matter what how high big the crowd becomes add to that the fact that you have "thugs" (enzymes) taking people (l-dopa/dopamine) out of the line.

think about the fact that 14mg of requip = 1000mg or more of C/L and assuming they are equal in 1 l-dopa molecule = 1 requip molecule, just shows how little 'l-dopa is needed by the brain.

so that's why you commonly read C/L doses above 250mg aren't any better.