There are reasons you would have shrinkage...in order of ascending likelihood..
1. overheated resin
2. overheated investment
3. insufficient pressure
4. insufficient duration of pressure
Barring these mistakes you should have accurate duplication.
I dont think that milling is a financially viable alternative.
BTW Snap on smile is a trademarked name. If youre not working with them.....pick a different phrase to describe your removable temporary veneers....or expect lawyers. They claim its patented.....though Ive not bothered reading the abstract to determine exactly what they patented.....
revised.....my curiousity got the better of me and I looked up
their patent. They patented the device not the process....and specified acetyl in the claims.
so again if your not working with them....You might want to try an alternative resin..or you might want to expect lawyers