Here's what Cornell has to say about it.

Here is a link that might be useful: tall spindle

As I wrote in my post, I've already read the Cornell info, as well as an Ontario Ag publication, which is very similar. I'm looking for input from growers with experience using other rootstocks. Thanks.

I apologize, I've gotten in the habit of not reading posts carefully.

I doubt you will get much help though, because the system is based on using dwarfing rootstocks to achieve very early crops and a vast reduction in pruning needs. Why are you considering using it for free standing trees?

i have managed espaliers where vigorous rootstocks were used and it really comes down to requiring spacing in ratio to the increased vigor. Just as important is to use varieties that are precocious and weak growing, of course.

MDFarmer

Some of the orchards around my area (SE WI) are planting
apples on M9 or derivative at 5' apart on trellis system.

A friend of mine in NW WI does the same thing but since
M9 not hardy for his area he plants on Bud-9.

My hobby orchard is on M7 and MM106 but I do have a few apples that are individually staked on M26 or Bud-9
(not sure as I did not buy these trees personally).

I am still unsure if I plant anything else if I would use
a trellis system or individually stake each tree if using
a very dwarf rootstock. some say a trellis is less expensive, yet others are staking each tree as afraid of
trellis failure long term. This is what I hear from some commercial growers near me.

I do know that I do not see any commercial orchards near me planting anything but super dwarfing rootstocks
at 4' and 5' spacing so that appears to be the future of
commercial growing in my area.

I also should mention nic-29 is being used too for trellis
systems around here. IF I do plant some on wire I believe I will be trying M9 as that seems to be the most popular here.

I have a small planting of tall spindle at 3 foot spacing which I top at 10 feet. I used a mixture of Bud 9, M9, G11 and G16, all of which have performed reasonably well.