This really won't have much effect on TNA/IW; at least, not to the extent that a lot of people here in the professional wrestling bubble will believe it will.
The Conventional Wisdom around here says that with UFC leaving, SpikeTV would reinvest heavily into TNA/IW. Strategically, that's putting all your eggs in one basket. Furthermore, there's absolutely no way that TNA/IW could produce the sheer amount of programming that UFC did. Think about it- Unleashed, which runs a couple of times a day, The Ultimate Fighter, which is getting Real Worldian in terms of its endurance and ability to still bring in ratings, along with the occasional televised live event. TNA/IW can't provide enough material to fill the gaps in UFC's schedule on their own- at best, they could provide a second night of programming and an Impact Wrestling rerun.
But the fact that TNA/IW can't provide daily programming (no professional wrestling company could) is going to work against them when it comes to providing just a second show. Who's going to fill the hole left by UFC? Executives at SpikeTV have spent all day trying to figure this out. More reruns of 'Manswers' and 'Jail' are a very obvious step backwards; if they're looking to build their channel rather than simply maintain it, they're going to be looking for another company to set up a tentpole show and fill that gap. As far as what that would be, I have no clue. It could be anything from an original sitcom to an attempt to get Rugby Union over with the American population.
What this means is that in the short term, TNA/IW enjoys an enhanced negotiating position with SpikeTV. If they want small favors done that they couldn't get done before- Dixie Carter's husband looks up expectantly- now's the time to ask for them. In the long term, however, this isn't likely to have too much of an effect. The only way TNA/IW gets a long-term benefit is if SpikeTV decides to go all-in on promoting TNA/IW; and I don't see them taking that kind of risk on what is still a niche audience. They'll be looking for someone else to diversify their programming.