Marinades actually only penetrate meat fractions of an inch, so while they do add flavor, they don't have a significant impact on the tenderness of the meat. (An overnight marinade isn't too long for this cut.) The cooking method and how you slice it are more important in that regard. Top round is a very lean cut, of course, and is best when seared, then grilled indirectly until it's medium-rare; or when it's braised low and slow in a flavorful liquid. Let the meat rest for a few minutes before slicing thinly across the grain. (This minimizes the length of the meat fibers.)
Steven, would you please comment on the use of needle tenderizing device to help marinade penetrate more deeply. My limited eperience with them in roasting or braising techniques make me think they might... but at the risk of over-needling and ending up with excessively mushy meat.
I think meat mechanical meat tenderizers like the one you descibe have a role in some preparations. In fact, that was one of the first things I added to my Best of Barbecue line of tools and equipment (see www.grilling4all.com). Potential uses? Flanksteak, pork shoulder steaks, tri-tip, brisket, top or bottom round, etc. They're quite handy, on occasion.