Rocket stove insulation

I have built stove of sorts. It is 5 inch steel tube ran up into a 30 gallon water heater. Drawn pic below. I did not insulate riser. I know this may not be the most efficient design, but it it what I have for now.
My question is, is it worth it to cut it open and insulate the riser tube? If so can I use metal ductwork for form and pour in perlite. Or should I use a mixed up fire mortar, perlite mix and a form that will burn away?
I can't rebuild it. This is my heat for now. So I just want to make it more efficient if it will till spring.
Thnx, Nathan

Sorry, apex dimensions are

The tubing is all 5 inch, 3\8 thick
Burn chamber 10 inches long
Feed tube 10 inches long
Riser is welded to 1\2 plate stand with clean out in very bottom
Riser is prob 40 inches tall, apex 30 above burn chamber entrance
Riser stops 3 inches from water heater at top
Water heater is apex 18x26
It burns OK
Exhaust is decent temp. I can touch the pipe where it exits water heater.
It leaves some lumpy pieces in the cleanout

You definitely want to insulate the heat riser, and the burn tunnel, possibly the feed tube. Without insulation you are losing a lot of heat from the main combustion zone, especially in such a small diameter system, and you will get less complete combustion (as evidenced by your mention of chunks in the cleanout).

Insulating a steel core will cause deterioration of the metal, but with a 5" system, the degree of heat may not be so severe that it will happen fast. It may last a couple of seasons before needing to be replaced, and you will have more experience when you do replace it. Be sure to inspect the interior regularly to see its condition.

You can probably just insulate around the existing riser with a perlite/fireclay mix, 2" thick or so. You can use light sheetmetal for an outer form. If the riser burns out, the perlite/clay will keep its shape; loose perlite would not.