Problem
You can slightly see the backbow if you follow the bass side fretboard edge
Solution
We recently had a 90s (if I remember correctly) ESP M-II come into the shop with an interesting problem. The neck had quite a severe amount of backbow, even with the truss rod nut all the way loosened, and the guitar strung up with a set of 11s. Despite the backbow being most severe at the 4th to 11th fret, where there was a decent amount of fret buzz, it caused playability problems further down the fretboard, where the action was very high. All in all, the guitar was unplayable because of this. Although this is a rather serious problem, it's not uncommon at all for older guitars that haven't been well maintained.
The solution we ended up going with was short of anything that required the fretboard to be removed. Since the frets on the guitar were still rather tall, we decided for a repair that focused on fretwork. We ended up doing a major fret leveling, focusing on the 4th to 11th fret. Most of the rough work was done with flat diamond plates before moving onto radius blocks. Since the backbow would have been more severe with the strings off, we figured that if we leveled the frets until they were perfectly level, that would give us a good amount of relief after the neck was strung up, bringing the truss rod nut back to use as well. After the frets were leveled and radiused, we continued onto crowning, dressing and polishing. With the truss rod just shy from needing force to tighten anymore, we strung up the guitar. With a set of 10s, we had about 0.030" of relief from the 1st fret to the body joint, which was perfect. And now the guitar can be setup for some low action and our client can keep shredding on!