If you are sucking in pink stuff with the engine cold, the t-stat will be closed and limit the amount of pink stuff getting past the t-stat and into the block. However, if you do not have any restrictions/valve on the bypass, that will allow some mixing of the pink stuff in with the raw water in the block even if the t-stat is closed, but, it will be diluted as compared to removing the t-stat, which should give a more thorough flooding of the block.
My $0.02.
