We just converted a tub drain to a shower drain and did a fair amount of research on venting, slope, etc. If the shower that was there before didn't have drain issues, there's not much you could do with that short of a run to create new ones, in my non-plumbing-expert opinion. If you're in a jurisdiction where a building inspection is required for your rough-in, you may get comments from that, though our experience has been that the building inspections aren't extremely picky. You might try testing the system before you enclose it. There are a couple of ways to test plumbing rough-ins if you want to Google for ideas.