Short answer: yes it does diminish resale value.
But here's another question - Why are you buying the guns? If you want them to fit well and shoot well then resale value isn't the issue. Besides, you can always keep the old stocks and put them back if youwant to sell.
A friend of mine had two 21s restocked to fit him perfectly and people kept telling him that it reduced their resale value. Well, he shot them both for well over a decade and when he died the resale value didn't really matter.
If you want, amortize the decrease over the time you're going to enjoy the guns and it becomes trivial.
