Well, I call them junk because that's what I think they are.

In spite of all the hoopla about high efficiency furnaces, there really is no magic way that a HE burns the fuel, as opposed to a lo-efficiency one. The difference in them is in how much heat they push out the chimney. It is your money, but in the past, there were a lot of the ceiling hanger types used here also. Gas was cheap, the shops were poorly insulated, and the hangers were also cheap. The combination served the purpose. Gas is no longer cheap and cutting firewood is hard work & lump coal is work and a mess so a gas furnace that uses most of the gas to heat the inside becomes a good thing.

It is probably too late to do this, as I'm sure that you're wanting to get the shop heated before it turns cold. If you get a cold snap before you do anything, I'd urge you to go to one of the places using a hanger and crawl on the roof and feel or measure the stack temp. Couple things work against the hanger when it comes to efficiency. One, the heat exchanger is too small for the size of the fire built inside. Two, the air they push over the exchanger is already very warm because it is coming from the top layer of air in the shop, and it is hot up there because heat rises. If you're in that shop during a cold snap get a ladder and crawl up there. It will be much toastier at the ceiling than it is on the ground. Just like the tractor radiator doesn't pull as much heat out of the coolant when it is hot outside, so too with the furnace - if it's moving hot air it can't transfer as much heat to it. A HE furnace will typically run plenum temps somewhere in the 90° range. Exhaust temps will be just a bit lower. Air that the hanger is working with will be hotter than that before it passes over the too small heat exchanger. The poor thing doesn't have a chance.