I've got a row of 20-30 pears on callery understock, most with an interstem of OHxF 513; probably somewhere between 5 & 10 years old now, some bearing fruit. Most, I can still 'sorta' pick out the graft union between callery, OHxF interstem, and scion variety... but on some, I can't see the difference... stem caliper is virtually identical, not much difference in bark character. But! there are a couple of them where the callery understock is of smaller diameter than the interstem or scion variety grafted onto them. Perhaps those are just less vigorous than the other callery seedlings I used.

Discussion linked here suggests 'overgrowth' not necessarily a problem. sion overgrowth