Gnosticism, Kaballah, MIdrash and Sensus Plenior all deal with riddles. Sensus Plenior is different in the results, the understanding of the nature of riddles, the methods for discerning expanded meaning, the sources used to solve riddles and particularly in the methods of constraining the meaning.
Gnostic methods produce a version of Kabbalah behind a thin veil of Christian terms. For the purpose of this classification, not all documents labelled Gnostic because they deal with riddles, are Gnostic. The Gospel of Thomas appears to be a tutorial for SP.
Midrash methods produce a philosphy of life such that the one who lives by it can make his good works outweigh his evil inclination.
Kabbalah methods produce a poltheistic occultic theology based in dualism of good vs. evil. There are some writers of Kabbalah who would deny this, but their writings still dabble in our ability to create reality with words, as in Magic.
Sensus Plenior always produces theology consistent with theology discerned using literal methods. Revealing Christ is always it's aim and result.
Nature of riddle
Kaballah, Midrash and Gnosticism violate the basic nature of riddle. They permit riddles to have many answers. However, if a Philistine had told Samson that the solution to his riddle was a mother bird which chews the sweet food before giving it to her chicks, Samson would not have conceded.
The only answer to a riddle is the one the author of the riddle intended. Sensus Plenior sees three basic purposes for its existence:
To inform the boy Jesus as to who he was and what he was to do. In this way, Jesus would not have to use omniscience to discern it. He would not take any advantage over us as he faced the same temptations that we do.
It would validate his works to the Bereans and those who wish to know that what he did was prophesied in great detail.
It would be kept hidden until the time when the Jews are grafted back into the vine. God will use methods they are familiar with, to let them see their Messiah in their scriptures.
In the meantime we can take great delight in meditating upon his word.
Methods of finding riddles
They all share the same methods for revealing riddles. Various formal lists are used such as the 32 Rules of Rabbi Eliezer, and Pardes. The same methods can be seen used by the Apostles.
Kabalah, Midrash and Gnosticism all use non-canonical sources to solve the riddles.
Kabalah addds books such as the book of Enoch, Midrash uses the writings and oral teachings of the rabbis, and Gnosticism uses sources from Greek Philosophy and the New Testament.
Sensus Plenior uses only the Old and New Testament. The solutions to all the riddles are found in Christ. Naturally Hebrew dictionaries and tools to understand language, help us to read the Hebrew text in an authentic manner. Kabalah and Midrash avoids the use of the New Testament.
Methods that constrain meaning
There are no methods to constrain Kabbalah, Midrash or Gnostic interpretations. Instead a primitive version of Stack Exchange is used where scholars vote on proposed solutions.
The rules which constrain meaning in Sensus Plenior are discerned from the scripture using sensus plenior (There may be more which haven't been discovered):
Since God has said that not a jot or tittle will pass away, until one knows why each jot and tittle is there, a complete understanding has not been derived. (This keeps us humble)
Since man shall live “..by every word”, a doctrine is not sound until it sums up and includes all that God has said about it. (This keeps us searching)
Since every word must be established by two or three witnesses, every shadow must have at least two supporting scripture witnesses. This means we cannot define a shadow with a single verse. (This keeps us rigorous in methodology)
Since God’s word is established forever, a shadow means the same thing everywhere is it used. So, since a donkey is a shadow of a prophet, everywhere there is a donkey, it is a shadow of a prophet. This rule alone makes the shadows humanly impossible to fabricate as it required the interlocking of a double entendre found in all the scriptures. (This keeps us in awe)
The riddle of Samson tells us Christ is the answer to all the riddles. If the shadow doesn’t look like Christ, it isn’t a good shadow. (This keeps us focused) 
And since we are to “let everyman be a liar and God be true”, outside references are not required to solve the riddles and see the shadows. (This keeps us devoted)
The Allegorical Method is based in Greek Rhetorical methods of invention. It has been used in the church to fill in the gaps where people have seen hints of sensus plenior but didn't know how to unpack it reliably. Because there are no controls, there have been many abuses of it.
 This point is also hinted at by rabbis: "Without pshat, and sod united, we cannot succeed in bringing Mashiah. For sod is the Mashiah", Rabbi Ariel Bar Tzadok, http://www.koshertorah.com/PDF/PardesLessons.pdf