I do not equate practicing a religion other than Mormonism - or, more broadly, Christianity - as being punishable sin. At its very worst, I see it perhaps as a Fall-induced transgression (since it will lead people to "break God's spiritual law" in some way or another) - but our Articles of Faith say that we will not be punished for those.
Years ago, I heard Elder Michelson say that he believes many members will be surprised in the end how many people receive a higher reward (including exaltation in the
Due to my experience, that statement resonated with me when he said it - and it resonates still. I agree with what he said, but I will add the explicit caveat that I believe what he said applies every bit as much to any Mormon as it does to any Protestant, Catholic, Muslim, Hindi, Buddhist, Jew, atheist or any other individual child of God. I believe all of these groups of people will be represented much more widely than many people believe in higher kingdoms than that to which we naturally would assign them.