When was the last time you had one of those fail?
I got the deck back in 2012...mind you, I don't run it 24/7, but I was using it a lot for awhile, then a bit of low use, and now will be ramping use up again.
When I got it I replaced the dozen burned out ones. Since then, only one more of the original bulbs has gone out, and none of the new/replacement bulbs.
So I don't see it as a real critical point, especially since each VU has two bulbs, and if one goes out, the other is still lighting the VU to an "ok" visible degree.
The new 6V bulbs are better and a bit brighter...so I would love if they all were the new bulbs, but I saw no reason to remove working original bulbs just for that.
My real point is that if the 8V bulbs are too dim, all you'll get is this sorta glow (kinda like my one meter with the single working bulb)...but from any bit of distance, there is not enough crisp brightness to really see the needle action.
That may not be an issue for some folks who have a different setup in the studio, or for someone who is purely engineering and is near the deck most of the time...but for me, when I step across the room to play an instrument or do a vocal, and I'm the only one there, being able to see what the meters are doing from a distance is important so I can properly set levels without having to go back-n-forth and all that.
My old TASCAM 3500 console was great for its bar graph meters, which were multi-colored and very bright and easy to see from a great distance...and since I would use the console for cue monitoring when tracking to the deck...if I couldn't quite make out the VU reading on the deck, I could glance over to the console.
Now the deck VU visibility is even more important because my current Trident console also has VU meters, and they are a bit smaller with no Peak indicator (like the deck has)...plus with the position of the console, its VUs are off to the side, and hard to see from many angles....while the old TASCAM bar graph meters you could see clearly from most any angle. Still...I'm happier with the Trident...but now I do depend on those VU meters on the Otari that much more...and TBH, I've thought of changing out the one bulb, 'cuz the one VU meter looks like it has a black eye...
...which I can actually do pretty easily by simply swapping out the whole VU with one of my spares, which is not as involved.
Anyway...the 8V bulbs may be OK...it's hard to know how much different the brightness is until you have both 6V and 8V bulbs installed in two VU meters, side-by-side...but I would still go for spec and brightness rather than worry about longevity, and like I said, the LED option doesn't appeal to me.