Ballistic Products doesn't sell 28ga slugs but do sell the round balls with special wad columns. They have load data in their book:
ht tp://www.ballisticproducts.com/Small-Bore-Manual-24-28-32-410-5th-ed/productinfo/00MSB/
Personally, I would see about finding round balls of the same size for cheaper and try replacing the special wad with a stack of fibre wads from someplace like Track of the Wolf. I've used BP load data for 12ga swapping the special wads they make with fibre ones to good result.
I'm sure someone has with round balls but I don't recall reading any details reports about them. Reviews I've read on threaded chokes, even extended ones, with factory sabot slugs are mediocre at best. If you have no other option or keep a threaded choke around for certain situations and don't or can't have a spare fully rifled barrel with you, I can see its viability. In most situations I'd rather have a fully rifled barrel if possible. Last I checked a decent rifled choke was around $80 and a fully rifled barrel for an 870 can be found for $200 used. I'd rather get the barrel than buy the choke, not be satisfied and have to buy the barrel anyway. My 870 came with two barrels, one fully rifled, so from the get-go I've had my preferred setup.
Also remember that the round ball or slug needs to be of the correct diameter to engage the rifling. Many 12ga round balls for example are .69" to .71" but the groove diameter on rifled barrels are usually .72" to .73" (they vary). Some people do get away with a smaller ball in a shot cup to fill the extra space but they are sometimes erratic. Most people theorize that there is slippage between the ball and cup. Lee foster slugs have the "key drive" piece in the base to try to prevent this for use in cups in rifled barrels (but I still get mediocre at best results in rifled barrels).