Before I sold the Pentax spotmeter I tested it against the new Sekonic 758DR by comparing reading from different objects in the room. Now and then I may have noticed a very slight difference which I attributed to not having one pointed from the exact same location as the other. But any difference was insignificant and otherwise each agreed with the other. Previously I compared the reading from the Pentax digital spotmeter with the Pentax 67II meter by comparing spot readings and average readings - taking several spot readings of a scene and averaging them. Both gave the same results.
Besides the batteries, the new 858 is able to read lower light thresholds which is useful in night photography.
According to a Sekonic product engineer who communicated with me in 2012 (and also mentioned in closing that both models were
"pending [sic]
in discontinuation"), neither the 758D or DR use identical or same metering systems for spot/reflected or incident. Further to this, the reading from the spot meter is
"below 18% gray" and
like [sic]
incident is also below the 18% gray reference". What he was saying is that the systems are not reading strictly to the gospel 18% grey, nor using one system but from later clarification, 13.5% spot, 15% incident.
This is why there is the ability to make very fine [+] or [–] baseline adjustments for either/both spot or incident. Additionally, this allows the meter's upper- and lower-end Ev to be adjusted to read in bright or low light, similar to the 858 you mentioned.