The possible issues with 8 bits and ProPhoto RGB are not related to dynamic range or color, but banding, especially in the shadows. Now, if you are done with the editings this risk is low.
I agree with Slobodan about converting to the printer color space. If they are requesting 8 bits, then they might also have specifications about color space (how do you softproof without it?). Converting to the final profile has also the advantage that you can select the rendering intent according to your taste.
In any case, make sure they handle properly a file in ProPhotoRGB
Yes, I fully agree with Slobodan and Francisco. If possible, use their recommended output profile for proofing and converting to that profile while in 16-bit/channel. Only then, also after resampling to output PPI and output sharpening, change the mode to 8-bit/channel for the final print file (which may then as well be a high quality JPEG, since no further image processing of any major significance is to be expected).