Mine have been vigorous, sometimes amazingly so for a dwarfing stock. Last yr I planted about 100 in March in greenhouse. Budded in June and trees are now 7ft mostly single stem scions spaced 6ft by 24 inches. A few have a couple fruit and all will bear next yr: apricot, pluot, nectarine.
I had a pluot on K1 for about 5 yrs that produced very high brix fruit. Had nectarine and peach 5-6 yrs. Smaller fruit but 24-28 brix and very good quality. The pluot had more vigor than nectarine. In fact the nectarine had very short internodes about like those genetic dwarfs.
K1 doesn’t root sucker but does send up shoots from any buds below scion including those under ground. If possible it’s recommended to debud the stock below ground before planting. No issue getting sprouts if scion fails on this stock.