ed, it's great that mucana is helping you. i keep putting off trying it but am going to!!
as far as long term survival taking C/L, here's an interesting article written in 2002. it came up on the first page of my search and if memory serves me i think it's conclusions are still valid. you basically live a lot longer than before C/L became available. brain studies on people taking C/L for a long time by people not having pd, treated for dystonia or other non-pd diseases or were misdiagnosed and whose substania nigra was not damaged, showed no measureable brain damage and some brains were autopsied. t'here's always a chance that the pd damaged brain could be damaged by C/L but the evidence points to pd, not C/L
these studies just look at cellular damage, not quality of life etc, but the paper is worth a read.

http://www.medscape.org/viewarticle/433387_3

a recent study was done using much larger qty's of carbidopa to test if carbidopa at qty's greater than 200mg can penetrate the BBB, which wouldn't be good since it would inactivate the enzyme in the brain that converts l-dopa to dopamine. you want this done while the l-dopa is outside the brain, not in the brain
Are high doses of carbidopa a concern? A randomized, clinical trial in Parkinson's disease - Brod - 212 - Movement Disorders - Wiley Online Library


here's another article about a study on patients taking more than 800mg/day showing the the 800mg limit by insurance companies should be reconsidered since patients did better. i think this is actually an 8 pill a day limit, not a mg limit.
Carbidopa/levodopa dose elevation and safety concerns in Parkinson's patients: a cross-sectional and cohort design -- Brodell
et al. 2 (6) -- BMJ Open