You need to find out the color management policies of the lab. A good lab will ask you to submit the files tagged with the working space you used to produce the file and can work with sRGB, aRGB, or ppRGB. They provide the profile for you so you can use it to soft proof, but they don’t expect you do to the conversion. Most prefer the file as a jpeg just because of size issues which means using ProPhoto isn’t advisable since it really does need to stay in 16bit. But some labs will allow you to supply the file as a 16bit ProPhotoRGB tiff file if you request it. This will depend on their ability to accept large files via transfer. We have customers that prefer this but they are local so they can bring them in on a DVD. If the output is through a chromogenic process there is no point in this, 8 bit aRGB or even sRGB has adequate gamut, but for a 9900, some images may see a slight benefit of staying in 16bit ppRGB.
The other question to resolve is the file resolution. We prefer our customers to provide the file in it’s original resolution, and we will do the resizing. We used to use a RIP for this, then migrated to Photoshop , but now I’m pushing for our normal process for this is to print directly from Lightroom. Old habits die hard and some of my employees I still catch using Photoshop, but recently I ask one to reprint it from Lightroom and they discovered the quality was better. It’s not they are not skilled in photoshop but Lightroom really is good at this and there really aren’t many people who have the skill to do better with other tools. We do have a few customers with a lot of experience and skill themselves and prefer to provide a resized file, and for the most part we can work with them.