My opinion: Cottered cranks are cool. Keep them. I agree with dck's observation that alloy rims would make more difference in ride quality.
But if you really want to do it here is a tip I found in Southerland's: an inexpensive Shimano bottom bracket can be mounted with Phil rings if you remove the fixed cup from the Shimano (using a vice and a plastic hammer). I have not tried this approach but it seems like a useful one to pass on to all you french-, raleigh-, and swiss-threaded bottom bracket owners looking for options.