Two of our three M10 RRs have the Uzi magwell conversion. I bought both guns that way, as they were already converted when we bought them. At $400 for the conversion, that is well within the range of pricing for the ease of sale. A M10 priced at $6,000 would sell very quickly, but more slowly at $7,000 just for the basic RR. So most would not care whether the gun, priced at $6,000, had the Uzi magwell conversion or not. Some would find the Uzi magwell conversion a plus, others would find it a minus. But at $6,000 those that want the conversion would leave it. And those that want the original magwell could have it switched and still be below market value of a M10 RR.
As posted above what gives the vast majority of the market value is being a transferable machinegun. Back when the market value of a M10 was $1,200, the cost of the Uzi magwell conversion would be more significant. Today the cost of an Uzi magwell conversion is not even 10% of market value. Maybe some day the factory configuration with original factory finish will make a significant difference in the market value. I don't see that happening anytime soon. A refresh of the finish is cheap. But selling the gun a couple hundred dollars would make up the difference IMHO. YMMV.