Ain't no thing but chicken wing. Adding a thumb insert/slug to a ball already drilled is no different than putting one in a ball when you first drill it. If you think it will help go for it. I have used slugs in every ball I have had since I was 14. They provide a uniform fit. To each his own
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The clock on the wall says 3 O'Clock...last call...for Alcohol!
"I love a martini -- but two at the most. Three I’m under the table; Four, I’m under the host." Dorothy Parker