Hi Mat
If you send the image to the printer as a tiff in ProPhoto with a good media profile, you'll get the widest gamut out of the printer that it can deliver. I have the luxury of printing from a RIP and send all mine (professional fine art reproductions) as 16-bit ProPhoto tiffs and let the RIP sort it all out, but that's as much for convenience as for any difference it might make to the final output - there's a recent thread here discussing the differences between 8-bit and 16-bit output, and the conclusion is that any visible differences there might be are exceptionally subtle.
Feel free to email if you need any further advice.
Best,
Malcolm