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5.0 grains of Bullseye is an old standard hardball loading from about WWI, so it could be historical bias. They may have seen some other kind of pressure sign with that combination and just decided not to push it any higher. The old copper crusher system has poor absolute accuracy and can be off by 25%, and that could account for it or at least their fear of it.
I note that Hornady's book takes a swaged 230 grain RN bullet up to 5.7 grains of Bullseye and hits 900 fps. Speer stops their swaged LRN at 5.0 grains, though, same as Lyman.
So chances are that you can work up from 4.5 to 5.0 grains and then creep up from there towards Hornady's number. Just do it a tenth of a grain at a time and keep an eye peeled for
pressure signs. If you see one, back down 5% and stop. Some powders can give you pressure spikes with some bullets. If you feel you really need more velocity, try a slower powder like Power Pistol.