Hi
The Supreme Court in Suraj Lamps & Industries Pvt Ltd. v. State of Haryana and another in (Special Leave Petition (C) No.13917 of 2009-Decided on [deleted]) had declared that there cannot be transfer of title by GPA/SA/WILLs transactions. Only registered sale deed /conveyance is a valid mode of transfer of title in immovable property.
So in your case, your Registered sale deed is the only instrument valid as per Transfer of Property Act.
The Supreme Court in the above mentioned case also emphasised that A power of attorney is not an instrument
of transfer in regard to any right, title or interest in an immovable property.
Also the Power of attorney itself is Invalid as Full Stamp duty has not been paid. So the Power of attorney cannot be regularised now by paying the deficit stamp duty.
So the answer to your query is
a) Your registered sale deed coupled with Possession gives you the absolute ownership rights/
b) Power of attorney is not a valid conveyancing deed subsequent to the judgment of supreme court and
c) more so in your case, the requisite stamp duty has not been paid and hence the document itself is null and void.
So no worries.