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Submitting to one of the OSM partners (such as Position Tech, Trellian, ineedhits.com, Marketleap, Infospider etc) will not boost your ranking.
It will get you into Yahoo! (and other search partners) quickly but it will not give you a ranking boost.
PFI does not improve ranking.
Remember that listing in the directory is per domain.
Listing in OSM is per URL.
But if you want to tweak it then the 48 hour refresh is a great way to get that happening. It provides with a short turn around time to see what impact the onsite changes have on your ranking.
Not only that, OSM will make sure you stay there for 12 months.
Being honest, the biggest benefit of OSM is getting the URL in their quickly. Unless you have the time, energy and skill to take advantage of the 48 hour refresh, then it may not make business sense.