I doubt that their largest contributers withhold much of anything from them, they would then have to find a way to take photos more efficiently to upload more since I suspect that their time is already maxed out.  For the top level contributers, I highly doubt (and Yuri's post backs this up) that SS makes up a very significant portion of their income. 

It will be interesting to see the aftermath of what becomes of IS's subscription plan.  To my eyes they are leveling a two pronged all out assault on the market in a major power grab; a direct attack by offering a favorable pricing plan for the most lucrative customers in the industry, attempting to lure them away from the competition, and outflanking their competition by stealing some of their best photographers.  Just like a battle, these two facets are mutually beneficial, more customers = more earnings for contributers, more earnings = more exclusives, more exclusives = competition's photo catalog hurt at the highest level (IS wouldn't be stealing noobs, they'd be stealing some of the best and most established photographers that contribute many of the premier photos in a collection), hurt the opponents photo catalog = more customers and the cycle continues.

IMO SS has to do something drastic, they will be harmed the most by IS's power grab, and I don't think that the tiered approach to payments is the answer, they have to find something else.  It is not the super top level earners that are the problem, it is the step below.  These guys I suspect is where the bulk of their broad quality comes from.  Higher quality than the noobs, not quite as much a the super top guys, but there are lot more of them than the super top guys.  These are the people that IS will be stealing.  The problem for SS is that losing these people will spread the earnings,that was concentrated in these mid level guys to the lowest people, more than shifting it to the super top, their customer base is already established in their areas of expertise.  This will make the ascension up the tiers more rapid for their lowest level contributers, who there are lot more of.  At first it wouldn't be significant, probably good for them actually, but over time as more and more ascend up the ranks, the payout per file would increase.

For example (using wild guesses for #'s, just to illustrate my point):
Currently:
50% sold at higher rate
50% sold at lower rate

Lose a bunch to exclusives, even though they gave a good raise to the top group and the situation becomes:
40% sold at higher rate
60% sold at lower rate

Because of the nature of tiers, concentrating more money in the lower tiers will rapidly become:
60% sold at the highest rate
40% sold at the lowest rate

Now this is assuming that IS can steal a bunch of exclusives, but the nature of SS leads to the fact the they have to out do both whatever the increase at IS will be, plus the multiplier for going exclusive.  As is (as I gather from all my reading here and elsewhere and personal experience), income growth at SS is rather stagnant, adding more is necessary just to maintain a level, which is not true elsewhere, adding more grows income.  Now you can increase the submission rate to SS to grow income, but the midlevel guys in the crosshairs are probably maxed out in their submission rate, they have the time and experience that their shooting efficiency is at a peak.  For these guys to see significant income growth at SS, the only way to accomplish it is via a big raise.

SS can only give so big of a raise to these mid level guys and remain solvent, they lack powerful financial backing unless they are currently running at a very excessive profit level.  IS has the financial backing an corporate stability to operate strategically in or near the red for a short period of time in an effort to offer a more lucrative deal to these mid level guys.  If income growth shows acceleration at IS, there will be takers for the exclusivity option, the more takers the worse the situation becomes over the long term for SS, as the acceleration up the tiers for their lowest level guys would begin.

Do I have an answer for what SS can do strategically?  No, I can't see one.  On a whole as a business their talent pool of photographers will swing increasingly more toward the beginners in the industry, and IS will steal some of their customers that follow the quality photos.  In many ways SS today reminds me of AOL in the mid 90's.  Their buisiness model allowed rapid ascension to near the top of the industry, but as both their customers and contributers grow into the young industry they find that there are better options available once established.  At first AOL was the internet for everybody, but over time they increasingly became the internet for beginners, eventually there reached a point where there were no beginners left.  I don't see that happening in this industry, there will always be beginners, but the dropping from a place for everybody to contribute to the place for beginners to contribute could very well happen, offering a tiered based raise would do little to solve this. 

SS needs to find a way with their buisiness model to maintain linear or near linear earnings growth for a mature port with continued contributions, something that is not occurring, the models at other sites that create the near linear growth for mid sized ports become more and more lucrative over time, and SS does not have the financial ability to make up this difference with a tier based pay system, since they would have to continually raise prices, and continually give larger and larger raises to the mid level contributers just to keep up with the competition for their services.