Great Power is doable. Superpower would require changing the definition or some bizarre theatrics.
If all the Iranian language speakers are in Persia, from the Kurds to the Tadjiks and Baluchs, then plausibly "Iranian" could come to mean "speakers of Iranian languages". It could feasibly retain control of Azerbaijan and maybe half of OTL Russian Central Asia. The Shi'ite states on the Gulf could be in Greater Iran's orbit (heck, they sort of are in our TL!). How about an alliance with a Shi'ite lower Mesopotamia, while we're at it?
This state could be true a 20th century Great Power, albeit a fairly delicate one - more on par with Italy, Japan, or Austria-Hungary than with the top tier Powers. Eventually, with luck and recognizable global trends, it would naturally surpass most individual European Powers, even Germany. But that's still not a superpower. The USSR would drastically outweigh such a state, even if reduced in size.
Bottom line, there simply aren't enough Iranian people in the world to make for a proper superpower. And there are hard limits to changing that, imposed by geography, rainfall, and soil. Getting Mesopotamia to speak something Iranian would be difficult without changing the numbers much; getting the Indus to switch language groups would require an epic ancient history POD, mess up all of modern history beyond recognition, and the region probably wouldn't stay in political union regardless.
It might take Persian settler colonies in political union, on top of everything. If you could get Persia to drop trouble makers and pretenders in Natal prior to 1500, say, just past the end of their trade routes to East Africa, perhaps South Africa could be dominated by ethnic Persians and converted to Islam. Then if Persia is the traditional protector of the region, maybe you could have eventual political reunion of one sort or another after the European irruption.
If a super-Persia dominates the Gulf and stretches from Kurdistan to the Aral Sea almost to the Indus, then it resurges in strength relative to the West with everything going perfectly right for a couple centuries (God protects fools, drunks, small children, and the United States of Iran?), then maybe. Then if a scattering of Shi'ite states from Walvis Bay to Kenya want to tie themselves to this Iran, and no power tries to drive a wedge in for some reason, and South and East Africa develop more than OTL....
That's a lot of maybe's, but it might be starting to resemble the power and influence of the USSR at its weaker moments.