I'm at the far end of the purist spectrum, so I would have left it as found. However, I do like those Timbersmith stocks; they're certainly the classiest aftermarket stock out there. The cold-bluing is the irreversible part but it does look surprisingly even and lustrous. How do you like the feel of the carbine with the stock and it's built-in cheek rest?
I am a bit of a purist too, thus the want to keep the bayonet. However the questions you ask are the reason for the stock swap. I am 6'2" so the longer length of pull is very welcome. The higher comb gives you a very good cheek weld that lines right up with the sights. For shooting it's a MUCH better stock than the original. However being a purist of sorts I did keep the original.
As for the bluing I see what you mean but this is not a collectors item where the original patina would matter. So many Yugo's and SKS's in general exist out there with perfect original bluing. Touchign this up so it looks near new I don't think changes it's value in the least. I'd go so far as to say if I didn't tell someone I did it that they'd never know or even question it. Now if it was a WWI 1903 or 1911 or some other war relic or even really old gun that had history then I would never touch that. In this case however I calculated that bluing it could only raise its value if anything.
Also I read several places that the TimberSmith is a pound or more heavier than the original. So I weighed the TimberSmith stock and handguard (+rubberband holding them together) BEFORE I routed out the groove. So mine is technically a tad lighter than this now that it's met my router;
After I removed the handguard from the gas tube I used the same rubber band and banded it to the stock and weighed it as well;
So the difference in my case is barely over a half pound and with the bayo channel removed maybe even slightly less than a half pound. However stock woods very greatly in weight and time in cosmoline combines with the type of stock wood can add who knows how much weight to any given stock so your mileage may very. All I am saying is that in my case the weight wasn't that different and well worth the extra shooting comfort.
...