Bluetooth Modules in a plastic enclosure
Generally with an RF application, you want to tune higher or lower than the frequency you are trying to broadcast. Th surrounding material attenuation then matches with the proper operating frequency. In this case you can not do that, so the range will drop. I believe you will still be fine for a few feet of range after looking at the datasheet and seeing you can crank it to 10 dBm. This also depends on the thickness of the case material however.

If you can find a module (or want to make one) with external filter components then you can switch these out and tune according to the case. Hope this helps.