hey ben, well ya, you got the whole 'metal is doomed' thing going, which is one problem, and RMH's are not well known for high performance with a 4" burn tunnel size, and I think the other problem might be in the mostly metal structure is radiating most of your heat too quickly. As you probably know, RMH's are specified to have firebrick for the burn tunnel, and for important reason. The firebrick absorbs the heat & holds the heat much differently than steel.
Most RMH's seem to always encapsulate the firebrick burn tunnel, & the feed tube areas with a thick application of cob, or whatever...which further slows down the ability for the firebrick of the burn tunnel to quickly radiate heat. Instead the combination of the firebrick & the thick cob overlay concentrates the heat in the burn tunnel & heat riser for a longer time as compared to metal. This concentration/insulation of heat in the burn tunnel & heat riser forces higher temperatures further down the line till it is allowed to exit the heat riser at maximum heat obtainable. The sudden transition from the 4" heat riser to the 12" barrel causes a change in the flow rate, slowing the hot air down somewhat.
You already got 450F out of it, which seems adequate for a shop heater, but for fun, I would like to see if you can get it near 650F. Remember the higher temperatures are what erode the burn tunnel faster, than lower temperatures...at 450F, that little stove should last for years, just like it is.
Just for fun, you could try dumping 2 or 3 big wheelbarrow loads of dirt around the support legs of the thing, mound the dirt pile up around the base, & sides covering the exposed metal lower parts of the unit. I would bring dirt & mound it up high, till it was to nearly 1" below the feed tube opening, therefore the whole bottom half of your unit would be embedded in dirt, including the exh. pipe area if ya want, this dirt pile might slow down your heat radiation in the lower parts of the unit, which is where you need to concentrate heat. This is just an experiment suggestion you might try. If you decide it is concentrating too much heat, some of the dirt mound could be easily pushed away...after all-- all you want to do is heat the shop, it is not necessary that you melt the burn tunnel out of it prematurely.
I think as your picture is now, it is your exposed metal pieces that are radiating the heat too fast, both internally & externally. I think the main goal of your style shop heater should be low chimney temperatures, therefore proving that you have extracted & used as much heat from the fuel you fed it. Generally if your chimney temperature is over 300, then more mass in/around your stove would help capture some of that excess heat.