"Is Jesus referencing John 15 fruit in Luke 8:14?" you ask. No, because while Jesus clearly speaks of fruit in John 15:1-8 (the fruit of the vine, namely, grapes) in his other parable in Luke 8:14 he speaks of seed. Further, of seed being planted in the ground, and whether it will eventually take root, grow, and then become multiple more plants. Also, in Luke Jesus very clearly states at the outset, "And this is the simile: the seed is the word of God." (vs.11 Y.L.T.)
Confusing two distinct parables will muddy the waters and not lead to clarity. Take each one, one at a time, and then learn their distinct lessons. This will also clear up you ideas about all those in both parables supposedly "being saved", as your comment about "the perseverance of the saints" indicates.
The Parable of the Seed in Luke 8 -
The illustration is of a farmer walking around, casting by hand seed from his basket slung round his neck. He is throwing handfuls of seed on to the ground, first with his left hand, then with his right, as he walks in a straight line. Jesus likened this to the word of God being cast abroad. He was showing that quite a lot of that seed would be wasted, insofar as it producing multiple more plants was concerned. It's never wasted, of course, as even those who reject it are judged by it.
"The word of God is alive and powerful, and sharper than any two-edged
sword, piercing even to the dividing asunder of soul and spirit, and
of the joints and marrow, and is a discerner of the thoughts and
intents of the heart." (Hebrews 4:12 A.V.)
Many times God's prophets and Christ's disciples have obediently spoken forth the living word of God into the world, only to be ridiculed, ignored, or to have people appear to be moved by it, only to show they had no root. The living word of God did not take root in them. But it always does grow in those whose hearts are right. In those ones, results are multiplied as the end of the parable shows:
"And that [seed] in the good ground: These are they, who in an upright
and good heart, having heard the word, do retain [it], and bear fruit
in continuance." (vs.15 Y.L.T.)
When the living word of God takes root in an upright and good heart, the proof of that is that that word of God then multiplies in the lives of others who the first person then shares the word of God with. They then share it with others, and that is how the Kingdom of God 'grows' in the world. Remember vs. 1 - Jesus was going through towns and villages preaching and proclaiming the good news of the Kingdom of God. He gave that parable of casting living seed around to help teach how the Kingdom grows. The word of God has to be cast abroad throughout the world.
Jesus being the True Vine in John 15:1-8 -
Here Jesus is addressing those who truly are his disciples, preparing them for his departure. He is not telling them how to spread around the good news of the Kingdom. He is warning them of their need to remain in him, even though he will soon depart. He is encouraging them to remain in him because he chose them:
"...and did appoint you, that ye might go away, and might bear fruit,
and your fruit might remain, that whatever ye may ask of the Father in
my name, he may give you." (vs.18 Y.L.T.)
The illustration here is of Jesus being the true vine. This is in contrast to "the vine of the earth" which angels cut down with a sickle, gathering up to be trampled in "the great wine-press of the wrath of God" on the Day of Judgment (Revelation 14:18-20).
The illustration Jesus used is of how to remain part of the true vine - himself. There will be some who appear to be part of him, but they will not be. Proof of that will be their inability to produce fruit. Fruitless vine branches are not rooted and grounded in Christ, but all who are remain in him and good fruit is seen in their lives. As Jesus said:
"In this was my father glorified, that ye may bear much fruit, and ye
shall become my disciples." (John 15:8 Y.L.T.)
This is not about the Kingdom of God growing but of how those who are already subjects of that Kingdom, whose King is Christ, must remain in Christ. Then they will show good fruit in their lives. Those who only appear to be Christians, but who are not "in" Christ in actuality, will not show good fruit in their lives. They might fool others, and even themselves, but they don't fool Christ! He is warning that they will be lopped off because they are not clean (vs.3).
Conclusion: What is the fruit Jesus refers to in Luke 8? Jesus is speaking about results. In Luke 8 the result of casting abroad the living word of God is growth in the Kingdom of God, but in the lives of many people hearing that word, it does not take root, so they show they have not become subjects of the Kingdom, therefore no spiritual increase results.
In John 15 Jesus is speaking about the result of remaining in him, the true vine. Such ones already belong to him and will show that by good fruit in their lives. Those only appearing to belong to him, but who actually do not, will not produce good fruit in their lives, and a time will come when they will be lopped off.
In no way does either parable cast doubt on "the perseverance of the saints" because "the Lord knows those who are his" - 2 Timothy 2:19. We have to concern ourselves with our belonging to the Lord, abiding in him, not judging others who we think might or might not be, because we are likely to be mightily mistaken. Further, Jesus assured us in John 6:39 that he would lose none of those the Father has given to him.