The problem is not duplicating the thumb in every ball. I have been drilling for over 27 years and that is not the problem. I have used custom thumb slugs for many years and now have converted to ovals which I can duplicate exactly in every ball. This has worked for many years until the last 1 or 2.
The problem is when your thumb changes, due to weather, lot of bowling etc. Agreed, everyone thumbs does not react the same way to these circumstances, but mine goes up and down like a yo yo. So if you have the exact thumb slug in every ball, then they all do not fit properly. That is the problem I am trying to address with converting to Switch Grip or something like it.
Having the option to insert a slug that is slightly bigger or smaller is the perfect solution. When you go from 1 piece of tape to 6 or more, for me, with 6 pieces or more, the thumb hole feels soft and not very comfortable. Not to mention that the top of the tape has a tendency to lift up. I know, some bowlers love tape and have no or very little problems using a lot but I am not one of them.
So, this really is not a ball drilling problem, just a physical thumb issue. I do live in the Northeast where the weather can be 0 degrees in January and 90 degrees in May. This does not help.
As far as changing your pitches with a solid switch grip insert, yes it is possible, but like already said it can not be too drastic of a change otherwise you will drill though the insert.
I have already ordered Switch Grip and Thoms Thumb. I need to do a little more research on Grip Loc before I order it. Like Steven and others said, I may have to try them all to see which one feels the best. I already have a few customers lining up to try them out with me. Should be a very interesting next few weeks.