As far as I'm concerned, a cue with turned down shafts doesn't diminish it's value, but that's because of the type of buyer I am. When I buy a cue these days, it's because I'm looking for my own personal playing cue, and not something to sell later. It's similar to cars in my opinion. If you have a car that has been altered from its original state with performance enhancers, lift kits, larger rims or whatever, the person likely to give you your money for it would be the guy looking to drive that vehicle. But take it to a dealer, and you won't likely get what you consider to be a fair price for it.
Same with cues that have been altered from their original state. If a guy is looking for a permanent player, then he is the buyer that you need to seek out 'cause the cue will be worth more to him than it would be to somebody trying to collect or flip it.
dave