I have not tried the new centers yet but I thought that this would just be a fun project. From what I understand, 2% shrinkage in aluminum casting is pretty common. You will get a little variance due to different alloys and final pour temps but I am guessing that it will be in the range of .01. I have heard that if you can control the final pour temp and Alloy, then you can get .001. I do not have these controls through.
The shrinkage can be easily overcome by increasing increasing the print size 2%. I have heard that hole sizes do grow as opposed to shrinking. I have only done one lost PLA cast (an unrelated item that needed high strength) so far. Consequently, I do not have a whole lot of experience with it yet but my first try was a success. I will have to play around with it to see.
There might not be any flex at all with the new centers. I just thought that this would be a fun project since I still need to print the new center anyway. Extra rigidity can never hurt as long as the added weight is not an issue.