The shield was there to prevent electromagnetic radiation escaping the device, just electrical radiation regulations really - the bit of writing on the back of the case refers to it.
"Complies with the limits for a class B computing device pursuant to Subpart J of Part 15 of EEC Rules"
FCC Part 15 is meant for "unintentional radiators". The category of unintentional radiators includes a wide variety of devices that contain clocks or oscillators and logic circuitry but that do not deliberately generate radio frequencies emissions - like a Gameboy.
It also works to help guide the cartridge into place and to prevent dust entering the body.
It isn't required by any means and originally didn't protect the cartridge from anything (and if it did, you aren't putting a Gameboy board in it!). I guess one issue you will have is light leaking from the SM LEDs on the Powerboost with the semi transparant case you have. That said you could always pop them off or something if this is an issue for you.
Removing the top half like you have seems like a really good idea actually - it gives a little more space and reduces the chance of it shorting the Powerboost. I might even go back and do the same to my first attempt
Hope that helps.
