...if you think this is the absolute maximum size this stock can get and any further growth will not be beneficial, that may indeed be true. I'm one of those fools that for someone reason has to experiment and see for myself however if any additional growth may indeed increase things like girth, size of leaves, etc. As my experience has often dictated that the smaller the stock the smaller the girth etc...
Oh I experiment too.
That is not the maximum size it can achieve. I grew one until it was over 5 feet tall with 4 main branches and a 3 cm trunk at the base, it was still growing and I only killed it because it had become very awkward and unwieldy to move around. Mainly because at the beginning I didnt plant it deep enough and it began to lean severely.
Its often argued that the typical pereskiopsis, like yours and mine, may not actually be the Pereskiopsis spathulata of Britton & Rose, but if it is they noted it as routinely growing to 6.6 feet tall.
As for ultimate height and girth of scion growth, I'd be surprised (and intrigued) if you havent already passed the point where those would be at a practical maximum.
I routinely see people use stocks way too short but with your two I wouldnt expect much improvement over stocks half that height.
If girth is what your after I do notice one thing. The leaf shape and spines on your two show that they have grown under half to full intensity sun. If I was after maximal girth of scions I would expect that to be produced more from intensity of light than height of stock, provided the stock was at least 1/3 the height of your two.
For ultimate over all size of scion growth, this is totally unverified and purely anecdotal, but I recently read an intriguing post by a thai grafter. We all graft to tender young new growth and pereskiopsis is temporary stock that fails after a year- give or take 6 months, its pereskiopsis 101, he asserted that if one took a grafted peres where the stock had turned 'brown', you know- that aged look with thick waxy cuticle they get after 6 or 8 months, and one degrafted the scion and grafted a new scion to the old woody pereskiopsis that the graft union would be semi-permanent. That the pereskiopsis would not reject the scion after a year or so like normal grafts do. Now, since the one I killed had grown into a bush with a 3 cm trunk its clear that a pereskiopsis base can support substantial mass if given the chance and if that guy was
actually correct about that grafting technique producing permanent or semi-permanent graft unions one could grow a HUGE scion on an 8 or 10 inch stock. I like to imagine a bowling ball sized Astrophytum held up by 3 stacks of bricks surrounding a four year old pereskiopsis stock
I dont claim that will work, but I do intend to try it some day!